Overview of ultrasound examination in obstetrics and gynecology

INTRODUCTION — Real-time sonographic imaging is the most common and most useful imaging technique employed in obstetrics and gynecology patients. However, it is a user-dependent imaging modality and multiple factors affect the images obtained, thereby directly affecting patient diagnosis and management. Operator experience and ability are probably the most important factors in making diagnoses that directly affect patient care. Other factors, such as the patient's body habitus, history of prior abdominal surgery, and, in obstetrics, fetal position, can all affect image quality and diagnostic performance.

Of note, over the past couple of decades, ultrasound equipment has progressively become smaller and is now available as laptop-sized and handheld systems [1]. These portable ultrasound systems can be used easily at the bedside (point-of-care ultrasound) and make ultrasound more available to users, especially in resource-limited settings, acutely ill patients, and emergencies [2-4].

This topic will provide an overview of the use of ultrasound examination in obstetrics and gynecology. The use of ultrasound for specific applications is discussed in individual topics on the specific disorders.

GYNECOLOGIC SONOGRAPHY

Indications — Gynecologic ultrasound examination has multiple uses, including, but not limited to, the conditions in the table (table 1) [5,6]. Use of ultrasound for these indications is discussed in detail separately. (See independent topic reviews for each condition, where available.)

Standard examination — The components of a typical gynecologic sonographic examination include [7]:

●Uterine size, shape, and orientation – A general rule for uterine size is that in a woman of reproductive age, the uterus is 8 x 4 x 4 cm in the absence of fibroids or adenomyosis, and can be an additional 1 cm in each direction in multiparous women. The mean outer dimensions of the uterus among 263 premenopausal women (nulliparas, primiparas, multiparas) and menopausal women (less than or equal to five years of menopause and greater than five years of menopause) with no uterine or ovarian pathologic findings are shown in the table (table 2). The average transverse width of uterine cavity (ie, inside dimension of the uterus) is 2.7 cm among nulliparas, 3.0 cm among primiparas, and 3.2 cm among multiparas [8].

●Appearance of the endometrium, myometrium, and cervix.

●Assessment of the uterus and adnexa (ovaries, fallopian tubes) for masses, cysts, hydrosalpinges, fluid collections, and uterine/ovarian mobility.

●Evaluation of the cul-de-sac for free fluid and masses.

Utility of screening examinations — In contrast to obstetric ultrasound, there is no role for routine ultrasound examination in asymptomatic nonpregnant women. The greatest area of interest in use of gynecologic ultrasound for screening is for ovarian cancer, with the goal of detecting malignancy at an early stage. However, in large randomized trials, annual transvaginal ultrasound did not result in a reduction of ovarian cancer mortality and had a high false-positive rate, which led to morbidity from surgery performed for benign lesions [9]. (See "Screening for ovarian cancer", section on 'Transvaginal ultrasound (TVUS)'.)

OBSTETRIC SONOGRAPHY

Timing and frequency — The American College of Obstetricians and Gynecologists (ACOG) has recommended ultrasound examination for all pregnant patients [10]. The timing and frequency depend on the indication for the examination.

●If a single screening examination is performed, the optimal time is at 18 to 20 weeks of gestation. This represents a time when fetal development and current ultrasound technology allow for the optimal detection of fetal anomalies. Additional important information from this examination includes assessment of the placenta and umbilical cord (eg, location, placenta accreta spectrum), confirmation of singleton gestation or diagnosis of multiple gestation, assessment of cervical length (screening for short cervical length), and assessment of fetal growth.

●If two screening exams are performed, the first is typically done in the first trimester, either at 7 to 10 weeks for reliable assessment of pregnancy dating or, in settings of expertise, at 11 to 14 weeks for nuchal translucency examination, pregnancy dating, and an early fetal anatomic survey (which can detect at least 50 percent of major fetal abnormalities by 14 weeks). Increased nuchal translucency is a marker for some aneuploidies and genetic syndromes, congenital heart disease, and some noncardiac anomalies. In either time period, the first-trimester examination also assesses cardiac activity, number of fetuses, and, in multiple gestations, chorionicity and amnionicity. In some countries (not the United States), pulsed wave Doppler examination of the uterine arteries is performed at 11+0 to 14+0 weeks to assess uteroplacental impedance as part of an integrated early screening test for predicting preterm preeclampsia [11]. (See "Enlarged nuchal translucency and cystic hygroma" and "Early pregnancy prediction of preeclampsia".)

The second screening examination is performed in the second trimester at 18 to 20 weeks for anatomic survey, growth, and pregnancy dating, as discussed above.

●If a third screening examination is performed in the third trimester, it is more pregnancy-specific with varied indications [12]. Major goals for all third-trimester scans are to assess fetal growth and amniotic fluid volume and screen for abnormalities that developed after or were not diagnosed on the 18 to 20 week examination. Common indications for third-trimester ultrasound screening examinations would include assessment of fetal growth in patients at risk for growth restriction or macrosomia, and assessment of the fetus for anomalies that are difficult to detect in the second trimester (for example heterozygous achondroplasia, where the femur length may not drop off the growth curve until the third trimester).

Third-trimester scans are typically performed between 32 and 36 weeks. Patients at high risk for fetal growth restriction often have two third-trimester screening examinations, one at 32 weeks and the other at 36 weeks. (See "Fetal growth restriction: Screening and diagnosis".)

In a retrospective cohort study of over 40,000 singleton pregnancies in a low-risk population that underwent standard sonography at 18 to 22 weeks of gestation with subsequent delivery of a live born infant, major anomalies were confirmed postnatally in 387 infants (1 percent) [13]. Among these infants, 281 were detected prenatally, of which 248 (64 percent) were detected at the initial fetal survey sonogram. Although one follow-up ultrasound improved the overall detection of anomalous fetuses from 64 to 71 percent, the yield was low in this low-risk population: 420 follow-up sonograms would need to be performed to detect an anomalous fetus not detected on the initial survey. In another large study, a previously undetected congenital malformation was incidentally detected in the third trimester in approximately 1 in 300 patients who were undergoing routine late pregnancy fetal growth assessment and had previous first- and second-trimester ultrasound screening [14]. The majority of these malformations involved the urinary tract, most commonly renal pelvic dilation, but also included neurologic abnormalities and skeletal dysplasia.

The natural history of some abnormalities limits the ability to dependably make a diagnosis at the 18 to 22 week fetal structural survey. Given the limited data, the current lack of clear practice recommendations, and the fact that some anomalies develop later in gestation and may not be identifiable at the first structural survey (eg, hypoplastic left heart syndrome, coarctation of the aorta [15,16]), follow-up structural surveys should be performed at the discretion of the obstetric provider and, if applicable, the consultant specialist. In the Paris Registry of Congenital Malformations, the anomalies most frequently missed prenatally but detected postnatally were genital anomalies; anomalies of the ear, neck or face; and limb anomalies [17].

Indications — The most common obstetric indications for ultrasound examination by trimester are listed in the table (table 3) [10,18]. Use of ultrasound for these indications is discussed in detail separately. (See independent topic reviews for each condition, where available.)

In high-risk settings, a more detailed fetal examination may be indicated. Indications for a more detailed anatomic examination include but are not limited to [19,20]:

●Fetus at increased risk for a genetic or congenital anomaly, based on past or present obstetric or family history of a genetic or congenital anomaly, maternal medical disorder (eg, diabetes, obesity), maternal teratogen exposure, conception by in vitro fertilization, increased nuchal translucency, multiple gestation, or abnormal antenatal blood screening

●Known or suspected fetal anomaly or known growth disorder (ie, early-onset growth restriction) in the current pregnancy

●Other conditions affecting the fetus, such as congenital infections, maternal opioid use disorder, alloimmunization, oligohydramnios or polyhydramnios

Nonmedical use — Because of safety and other issues, there is consensus among national/international medical societies and regulatory bodies that prenatal ultrasonography should not be performed for nonmedical reasons, such as solely for parents to have a keepsake picture/video of the fetus, to learn the sex of the fetus without a medical indication, or for commercial demonstration purposes such as trade shows [10,21,22]. Likewise, Doppler ultrasound stethoscopes are available by prescription and should not be provided to parents to listen to the fetal heartbeat at home without a medical indication and guidelines for use [21]. (See 'Safety' below.)

ACOG has also added that nonmedical ultrasonography may falsely reassure women. Abnormalities may not be detected in these settings. Furthermore, these nonmedical providers are not prepared to discuss and provide follow-up of worrisome findings [10].

Utility of routine examination — Many meta-analyses of randomized trials have evaluated the utility of routine prenatal ultrasound examination. The included trials were generally of low or moderate quality. In particular, the late pregnancy trials used diverse definitions of screen positive, were designed in the absence of high-quality data on diagnostic effectiveness, did not couple screening to an effective intervention, and were underpowered for some outcomes (eg, perinatal mortality) [23]. With these limitations, the following findings have been reported:

●Compared with selective use, routine first-trimester ultrasound examination did not result in clear reductions of induction of labor for postterm pregnancy (RR 0.83, 95% CI 0.50-1.37) or perinatal loss (defined as miscarriage, termination of pregnancy, intrauterine death after trial entry, or death of a liveborn infant up to 28 days of age or before hospital discharge: RR 0.97, 95% CI 0.55-1.73), but appeared to reduce short-term maternal anxiety about the pregnancy (RR 0.80, 95% CI 0.65-0.99) [24]. However, the overall quality of data was not high given that the trials in the analysis were mostly from relatively early in the development of the technology, many control participants also had scans, and the trials were underpowered to show an effect on other important maternal or fetal outcomes.

●Compared with selective use, routine second-trimester ultrasound examination reduced the chances of induction of labor for postterm pregnancy (1.4 versus 2.8 percent; RR 0.48, 95% CI 0.31-0.73) and not detecting a multiple gestation by 24 to 26 weeks (RR 0.05, 95% CI 0.02-0.16) [24]. It also increased the changes of detecting a fetal anomaly by 24 weeks (RR 3.45,95% CI 1.67-7.12). However, there was no clear reduction in perinatal loss (RR 0.98, 95% CI 0.81-1.20). The overall quality of data was not high given that the trials in the analysis were mostly from relatively early in the development of the technology, many control participants also had scans, and the trials were underpowered to show an effect on other important maternal or fetal outcomes.

●Compared with selective use, routine ultrasound examination in late pregnancy (after 24 weeks of gestation) did not significantly reduce perinatal mortality (RR 1.01, 95% CI 0.67-1.54), preterm delivery (RR 0.96, 95% CI 0.85-1.08), or induction of labor (RR 0.93, 95% CI 0.81-1.07) but was not clearly harmful as it did not increase cesarean delivery rates (RR 1.02, 95% CI 0.97-1.09) [25]. Potential psychological effects and other neonatal outcomes could not be assessed.

Similarly, compared with serial fundal height measurements, routine ultrasound examination in the third trimester did not reduce perinatal death in low-risk pregnancies (0.4 versus 0.3 percent, RR 1.14, 95% CI 0.68-1.89) [26].

●Compared with no Doppler ultrasound, routine Doppler ultrasound in unselected low-risk populations did not significantly reduce perinatal mortality (RR 0.80, 95% CI 0.35-1.83) or serious neonatal morbidity (RR 0.99, 95% CI 0.06-15.75) [27]. Routine use of Doppler of fetal vessels/umbilical artery may have improved rates of stillbirth (RR 0.34, 95% CI 0.12-0.95). By contrast, a reduction in perinatal death has been observed in high-risk populations (RR 0.71, 95% CI 0.52-0.98) [28].

Scope

Standard examination

●4.5 to 8 weeks – An obstetric ultrasound examination between 4.5 and 8 weeks shows the size, location, and number of gestational sacs; the yolk sac at approximately 5.5 weeks and the embryo at approximately 6 weeks; and cardiac activity thereafter.

In many multiple gestations, chorionicity and amnionicity are easily determined with high confidence at seven to eight weeks of gestation. In monochorionic twins, however, the amnion at seven to eight weeks is closely adherent to the embryo and may be difficult to detect.

●9 to 11 weeks – As the early pregnancy transitions from the embryonic to the fetal stage of development, more anatomy is identifiable sonographically. Some findings, such as the physiologic herniation of bowel, are normal and resolve by the end of this period. Some abnormal findings, such as holoprosencephaly, are identifiable during this time.

●12 to 14 weeks – The specific components of late first- versus second- and third-trimester examinations are described in detail in the American Institute of Ultrasound in Medicine-American College of Radiology-American College of Obstetricians and Gynecologists-Society for Maternal-Fetal Medicine-Society of Radiologists in Ultrasound (AIUM-ACR-ACOG-SMFM-SRU) Practice Parameter for the Performance of Standard Diagnostic Obstetric Ultrasound Examinations and the AIUM Practice Parameter for the Performance of Detailed Diagnostic Obstetric Ultrasound Examinations Between 12 Weeks 0 Days and 13 Weeks 6 Days, which are beyond the scope of this topic [18,29]. The ISUOG also has practice guidelines for performance of the 11-to 14-week ultrasound examination [11].

The detailed examination can be performed in the late first trimester in women at increased risk for fetal or placental abnormalities potentially detectable at this time [29].

●>14 weeks – Standard obstetric ultrasound examination in the second and third trimester generally provides the following information [10,18,19,30-32]:

•Fetal number (chorionicity and amnionicity if multiple gestation). (See "Twin pregnancy: Overview", section on 'Assessment of chorionicity and amnionicity'.)

•Fetal biometry – Fetal biometry is used to estimate gestational age and fetal weight, as appropriate for the stage of pregnancy. (See "Prenatal assessment of gestational age, date of delivery, and fetal weight".)

•Fetal presentation/malpresentation. (See "Overview of breech presentation", section on 'Imaging'.)

•Documentation of fetal cardiac activity, which may include rate and rhythm. (See "Fetal arrhythmias".)

•Placental appearance and location, including the site of cord insertion into the placenta. (See "Placenta previa: Epidemiology, clinical features, diagnosis, morbidity and mortality" and "Placenta accreta spectrum: Clinical features, diagnosis, and potential consequences" and "Velamentous umbilical cord insertion and vasa previa".)

•Assessment of amniotic fluid volume. (See "Assessment of amniotic fluid volume".)

•Survey of fetal anatomy (calvarium, lateral ventricles, choroid plexus, falx cerebri, upper lip, nuchal region, four-chamber cardiac view, three-vessel trachea view, stomach, kidneys, bladder, cord insertion site at abdomen, limbs, genitalia in multiple gestations [and when medically indicated]). (See individual topic reviews for each anatomical region)

In a study of early sonographic diagnosis of fetal sex that included over 2100 nonanomalous fetuses, accurate sex determination was often possible beginning at about a BPD of 18 mm (11.7 weeks), but the authors believed that fetal sex assignment should not be undertaken below a BPD of 22 mm (12.8 weeks) in cases where fetal sexing affected pregnancy management [33].  

In some cases, the genital appearance may be atypical. (See "Evaluation of the infant with atypical genital appearance (difference of sex development)".)

•Maternal anatomy (cervix, uterus, adnexa, cul-de-sac). ACOG recommends examining the uterus, cervix, and adnexa when technically feasible [10]. AIUM and ACR affirm this position and further state that "a transperineal or transvaginal scan may be considered when evaluation of the cervix is needed" [34,35]. (See "Short cervix before 24 weeks: Screening and management in singleton pregnancies".)

•Fetal movement. Temporary absence or reduction of fetal movement during an examination is not necessarily worrisome as it can be due to a normal fetal sleep cycle. (See "Decreased fetal movement: Diagnosis, evaluation, and management", section on 'Normal fetal movement' and "Biophysical profile test for antepartum fetal assessment".)

In-depth discussions of sonographic examination of the placental abnormalities, amniotic fluid volume, fetal growth abnormalities, fetal markers of aneuploidy, specific fetal anomalies, and other fetal and placental disorders can be found separately in individual topic reviews on each subject.

Limited examination — Limited ultrasound examinations can be performed to address specific focused questions when an immediate impact on management is anticipated and when time or other constraints make performance of a standard examination impractical or unnecessary [30,36]. Examples of appropriate use of limited studies include confirmation of the presence, size, location, and number of gestational sac(s); presence or absence of fetal cardiac activity; checking fetal presentation; measuring cervical length; and assessment of amniotic fluid volume in conjunction with nonstress testing or assessment of fetal well-being with a biophysical profile [10,34,36].

Ideally, a limited examination is performed in patients who have been previously evaluated by a complete examination. In women who have not previously had a standard or detailed ultrasound examination, a standard or detailed ultrasound examination should be obtained when and where appropriate.

Second- or third-trimester detailed examination — A detailed (or specialized or comprehensive) fetal structural survey should only be undertaken by those with the necessary training and skills required for these advanced examinations. Indications for a detailed fetal examination include, but are not limited to, a previous pregnancy affected by a fetal anatomic or chromosomal abnormality; suspected, including those at increased risk for, or known fetal anatomic or chromosomal abnormality in the current pregnancy; known fetal growth disorder; and current pregnancy complications possibly affecting the fetus (eg, congenital infection, abnormal amniotic fluid volume, alloimmunization, suspected placenta accreta spectrum) [19,31]. Fetal evaluation in these settings requires a more detailed examination of fetal and placental anatomy than a standard fetal survey and requires more advanced skills and knowledge.

In 2013, a task force composed of participants from several major national obstetric and radiologic organizations in the United States developed a consensus report with guidelines for performance of this examination [19]. The detailed examination includes all of the elements of the standard examination. In addition, there are specific elements beyond those in the standard examination that are individualized and determined by the indication for the examination, the ultrasound findings identified during the examination, and the specialized knowledge and training of the responsible physician [31]. These elements may include evaluation of:

●Skull and intracranial anatomy, including all ventricles, choroid plexus, parenchyma, cisterna magna, cerebellar vermis, and corpus callosum

●Neck, including nuchal thickness

●Inner and outer facial structures (eg, profile, palate, orbits, lens, nose/nasal bone, jaw)

●Ears

●Cardiovascular anatomy (detailed evaluation beyond four chambers, outflow tracts, and three vessel thoracic view)

●Thorax, including diaphragm and lung masses/cysts, ribs

●Small and large bowel

●Adrenal glands

●Gallbladder, liver

●Spleen

●Kidneys and renal arteries

●Spine

●Limbs, including hands, feet, digits (number and position), humerus, femur, ulna, radius, tibia, fibula

●Genitalia

●Umbilical cord (structure)

●Placenta (lobes, masses, abnormal adherence)

●Neoplasms

●Expanded biometry

●Maternal anatomy

Safety — No significant adverse effects have been identified in children exposed to obstetric ultrasound examination in utero and followed for several years after birth. (See 'Evidence of effects in humans' below.)

Theoretical concerns about thermal effects, cavitation, and vibration — The primary concerns of the use of ultrasound technology are with respect to thermal effects of the insonated tissue and cavitation of tissue due to the production of gas-filled bubbles [37-39]. The mechanical index (MI) is an estimate of the compressive and decompressive mechanical effects of ultrasound pulses (which can potentially result in cavitation), and the thermal index (TI) is an estimate of the degree of temperature elevation.

The temperature increase with diagnostic ultrasound is less than 1°C at typical acoustic output levels as long as the TI is maintained below 1.0. This level of increase is not felt to be clinically significant. Similarly, diagnostic ultrasound used for medical imaging does not appear to cause cavitation at usual obstetric acoustic output levels as long as the MI is kept below 1.0 [39,40].

The ultrasound wave affects the tissue through which it travels by mechanical vibration and heating of this tissue. Mechanical vibration can result in cavitation or the formation of gas bubbles. This is an effect that typically occurs at the interface of tissues and gas. Because there is no gas within the gestational sac, and specifically in the fetal lung or bowel, this is thought not to be a significant factor in obstetric sonography at the currently used levels of diagnostic sonography. Nevertheless, keeping MI <1.0 is good practice.

Thermal effects have the greatest potential for adversely affecting the fetus. Routine B-mode, as is used for typical two-dimensional (2D) imaging, does not increase the temperature above the 1.0 to 1.5°C range that is thought to be safe for fetuses, especially those early embryos at the time of embryogenesis. Spectral Doppler ultrasound and, to a lesser extent, color Doppler ultrasound, however, use higher energy and focus the acoustic energy that is created on a much smaller volume of tissue than typical 2D imaging does and can result in changes in tissue temperature, especially at bone-tissue interfaces. For this reason, Doppler ultrasound should be used with great care, especially early in pregnancy, in spite of the lack of significant calcification of bone at this time [37,41].

The intensities used in transvaginal examinations are generally lower than those in transabdominal examinations. However, thermal risk can only be assessed by determining the TI in each case.

Evidence of effects in humans — In 2009, the World Health Organization (WHO) systematically reviewed 61 publications reporting data on the safety of B-mode or Doppler ultrasound in human pregnancy [42]. These data showed that ultrasonography during pregnancy was not associated with adverse maternal/fetal/neonatal outcome, impaired physical or neurologic development, increased risk of childhood malignancy, impaired cognitive ability, or mental disease. There was an unexplained weak association between ultrasound and non-righthandedness in boys. The weak association with non-righthandedness has also been reported in a meta-analysis of follow-up data of 8865 children ages 8 to 14 years from three randomized trials on routine ultrasonography at 15 to 20 weeks of gestation [43].

There were limitations to the WHO data, which prevent a firm conclusion about safety [44]. For example, the studies were observational and assessing bioeffects was not the primary objective, the intensity of ultrasound exposure was not usually measured, ultrasound technology changed over the period these studies were performed, and subtle and longer-term changes could have been missed. Nevertheless, the data are reassuring. In 2022, the AIUM noted that some older studies reported that fetal exposure to diagnostic ultrasound appeared to be associated with an increased risk of low birth weight, delayed speech, dyslexia, and non-right-handedness, but also noted that more recent studies have not demonstrated such effects despite higher maximum recommended levels of acoustic output for ultrasound machines used for fetal/obstetric applications [45]. As of this writing, there are no known deleterious effects of ultrasound when operating the machine using standard guidelines. Nevertheless, the overarching goal in pregnancy should be strict adherence to the ALARA (As Low As Reasonably Achievable) Principle using ultrasound with the lowest acoustic output possible for the least amount of time possible to acquire diagnostic images.

Pregnancy-related risk factors for development of autism spectrum disorder in childhood is an active area of investigation. No studies have found an association between the timing or frequency of prenatal ultrasound exposure and risk of autism spectrum disorder [46-48].

Performance guidelines to maximize safety

●The use of ultrasound should be reserved for clear medical indications.

●B-mode and M-mode imaging operate at acoustic outputs that do not produce harmful temperature rises; therefore, temperature increases are typically not a concern. Nevertheless, scans should be performed over the shortest period of time and with the lowest energy output possible to permit adequate diagnostic accuracy [49,50].

●In contrast to B-mode and M-mode, the use of spectral and color (including power) Doppler ultrasound diagnostic equipment has greater time-averaged intensities and, therefore, the potential to produce biologically significant temperature rises, particularly in the vicinity of bone, which has high acoustic absorption and the potential for heating adjacent tissues. It is particularly important that sonologists who perform ultrasound imaging of first-trimester pregnancy understand the TI and MI acoustic output indices involved [51]. The potential for heating is also high when intracranial Doppler studies of the middle cerebral artery are performed in the second and third trimesters in fetuses at risk for anemia. The sonologist may interrogate a single location for a prolonged period of time during these studies, and since the location is near a bone tissue interface, it is important to pay attention to dwell time and move the probe frequently. (See 'Theoretical concerns about thermal effects, cavitation, and vibration' above.)

Acoustic output is under the control of the operator. Use the minimum output and duration of Doppler insonation consistent with obtaining the required diagnostic information on the fetus. Altering the focal depth or the size of a box of interest or a Doppler gate will change the TI and MI. In addition, the effects of elevated temperatures may be minimized by keeping the time during which the beam passes through any one area as short as possible.

The International Society of Ultrasound in Obstetrics and Gynecology Clinical Standards Committee developed practice guidelines and consensus statements to provide health care practitioners with a consensus-based approach for diagnostic imaging [52,53].

•First-trimester use of fetal Doppler is a particular concern because of potential effects on organogenesis. Spectral Doppler, color flow imaging, power imaging, and other Doppler ultrasound modalities should generally be avoided in the embryonic period (up to 10+6 weeks of gestation). If information from Doppler is needed this early in gestation for pregnancy management, the exposure time should be kept to a minimum. As of 11+0 weeks, organogenesis is complete and the fetal-placental circulation is established so Doppler ultrasound modalities may be used for screening for specific clinical indications, such as screening for trisomy and cardiac anomalies. The TI should be ≤1.0 and exposure time should be kept as short as possible, usually no longer than 5 to 10 minutes and not exceeding 60 minutes. The minimal power output and duration of Doppler insonation required to obtain diagnostic information from the fetus should be used.

•Doppler interrogation of maternal uterine arteries may be performed at any point in the first trimester without fetal safety concerns as long as the embryo/fetus lies outside the Doppler ultrasound beam.

●Ultrasound should be used with caution in febrile mothers because of the increased risks of potentially harmful heating [54,55].

●The use of contrast agents in obstetric patients should be avoided because of the risk of nonthermal cavitational effects and the lack of data demonstrating efficacy and safety. Ultrasound contrast should not be used in pregnancy even in the absence of the use of Doppler [54,55].

BASICS OF CLINICAL USE

Personnel — Sonography is an operator-dependent technology: A high level of proficiency can only be achieved by supervised experience with a large variety of normal and abnormal examinations.

In the United States, ultrasound examinations in obstetrics and gynecology are typically performed by diagnostic medical sonographers. Almost all of these health professionals are credentialed by the American Registry for Diagnostic Medical Sonography (ARDMS) and receive the credentials "RDMS" upon completion of extensive education, training, and testing. Some sonographers are also credentialed by the American Registry of Radiologic Technologists.

The examinations are supervised and interpreted by a sonologist, who is a physician with training and experience in this area. It is up to the physician to provide a report based on the data provided by the sonographer and, if indicated, by personally scanning the patient to confirm or modify the differential diagnosis. In the United States, the American Institute of Ultrasound in Medicine and the American College of Radiology have formulated guidelines for training, credentialing, continuing education, and ultrasound laboratory accreditation (available on the internet at www.aium.org and www.acr.org).

Ultrasound modalities

●B-mode and M-mode – B-mode imaging, or brightness mode, is the most common technique for imaging in obstetrics and gynecology. M-mode, or motion mode, is useful for evaluating the fetal heart (eg, rate and rhythm, valve motion). (See "Congenital heart disease: Prenatal screening, diagnosis, and management" and "Fetal arrhythmias".)

Almost all obstetric and gynecologic sonography is done in real time, freely moving the probe to view each structure from multiple orientations. It is primarily used to evaluate morphology and is especially useful for studying the anatomy of moving objects, such as the fetus, or for evaluation of the movement of structures, such as the ovaries and cul-de-sac, with suspected endometriosis. Biometric measurements are typically performed on the display monitor from a frozen image.

Real-time evaluation of fetal movement, breathing, and tone form the basis of one of the two main tests of fetal well-being: the biophysical profile. (See "Biophysical profile test for antepartum fetal assessment".)

Real-time imaging is also used to guide invasive obstetric procedures, such as amniocentesis, chorionic villus sampling, and fetal blood sampling and/or transfusion. (See "Diagnostic amniocentesis" and "Chorionic villus sampling" and "Fetal blood sampling" and "Intrauterine fetal transfusion of red blood cells".)

●Doppler ultrasound – Doppler ultrasound is used for studying most of the major fetal circulatory systems and is particularly helpful in evaluating the functional state of the fetal cardiovascular system, fetoplacental and uteroplacental blood flow, and pelvic cysts and tumors. Several modalities are available (continuous wave, pulsed wave [spectral Doppler], color flow mapping, power). The choice depends on the information needed (eg, fetal heart rate versus velocity and direction of blood flow); more than one modality can be used.

Specific clinical uses of Doppler ultrasound include, but are not limited to, the following:

•Assessment of fetal well-being in pregnancies complicated by fetal growth restriction. (See "Fetal growth restriction: Pregnancy management and outcome", section on 'Fetal surveillance'.)

•Evaluation of fetal anemia by assessment of peak systolic flow in the middle cerebral artery. (See "RhD alloimmunization in pregnancy: Management", section on 'Assess for severe anemia using MCA-PSV in fetuses at risk'.)

•Evaluation of the maternal-placental interface in the diagnostic evaluation of the placenta accreta spectrum. (See "Placenta accreta spectrum: Clinical features, diagnosis, and potential consequences", section on 'Color Doppler'.)

•Evaluation of suspected vasa previa. (See "Velamentous umbilical cord insertion and vasa previa", section on 'Imaging'.)

•Assessment of blood flow to suspected ovarian and endometrial tumors. (See "Adnexal mass: Ultrasound categorization" and "Overview of the evaluation of the endometrium for malignant or premalignant disease", section on 'Transvaginal ultrasound'.)

●Advanced techniques – The following clinical techniques are used selectively to provide further evaluation of uncertain findings on real-time grey-scale two-dimensional (2D) sonography or allow a more expansive evaluation not available with standard grey-scale sonography [56].

•Three-dimensional (3D) sonography – The use of this technique can reduce scanning time while maintaining adequate visualization of the fetus in obstetric ultrasound and the pelvis in gynecologic ultrasound [57-59]. Surface rendering of the fetus with 3D sonography can better demonstrate abnormalities previously detected with 2D sonography, especially facial, skeletal, and central nervous system abnormalities.

In gynecology, the coronal plane of the uterus is easily obtained with 3D but not 2D sonography, thus enhancing visualization of the uterus, especially the uterine cavity. This has improved the ability of sonography to diagnose uterine anomalies [60] and identify malposition of intrauterine devices [61].

•Four-dimensional (4D) sonography – Four-dimensional sonography (also called dynamic 3D sonography) refers to 3D images that can be viewed in real time. It has been used to study the fetal heart, fetal movement, and fetal behavioral states.

•Sonohysterography – Saline infusion sonohysterography refers to a procedure in which fluid is instilled into the uterine cavity transcervically to provide enhanced endometrial visualization during transvaginal ultrasound examination. Rarely, the examination is performed transabdominally, such as when the uterus is lifted out of the pelvis, usually by fibroids and less commonly by adhesions. (See "Saline infusion sonohysterography".)

As an extension of sonohysterography, hysterosalpingo contrast sonography (HyCoSy) is particularly useful for evaluation of tubal patency. Although contrast agents can be used, saline with "microbubbles" (ie, saline agitated with air) can provide valuable information noninvasively [62]. (See "Female infertility: Evaluation", section on 'Hysterosalpingo-contrast sonography'.)

•Elastography – Elastography assesses the elastic properties of tissues and is an area of active research in obstetrics and gynecology. Although not currently standard of care, investigative work has included evaluation of the cervix for assessment of risk for preterm birth [63] among many other areas of obstetrics and gynecology.

•Slow-flow microvascular Doppler ultrasound – Slow-flow microvascular Doppler ultrasound is a new technology that identifies slower flow in smaller vessels. It appears to be safe in obstetrics as safety indices are well within recommended values [64]. Given its recent availability, its use in obstetrics and gynecology requires further study.

Patient preparation

●Reason for the examination – The sonographer should know the indication for the ultrasound examination and results of other evaluations related to the patient's problem. The last menstrual period (or estimated delivery date in obstetric examinations) should be documented. All of this information is critical for targeting specific structures, choosing whether to use a transvaginal and/or transabdominal technique, and deciding whether additional studies may be helpful (eg, saline infusion sonohysterography, Doppler velocimetry).

●Patient position – In obstetrics and gynecology, most examinations are performed with the woman in a semi-recumbent position. A padded table and pillows provide reasonable comfort. It is desirable to be able to elevate the head of the bed because many pregnant women are unable to lie flat, especially later in pregnancy. Others will require pillows under their knees or behind their back to achieve a comfortable position.

Transvaginal ultrasound examinations are done with the woman in a lithotomy position. Alternatively, a cushion can be placed under the buttocks to raise the pelvis, while the lower extremities are separated and/or frog-legged (soles of feet together and knees apart).

●Modifications for patients with obesity – Abdominal obesity limits the technical quality of the ultrasound examination. Imaging may be improved by having the patient lie on her side and placing the transducer at the side of the maternal abdomen rather than in the midline where the thickness of abdominal adipose tissue is often greater, and/or by use of transvaginal ultrasound.

Although fetal anatomic surveys for malformations are typically performed transabdominally at 18 to 20 weeks, performing the examination later in gestation (20 to 22 weeks) in the obese gravida may improve visualization of anatomy [65]. Transvaginal ultrasound in the late first or early second trimester may also help with fetal evaluation in these patients [66].

●Bladder filling – In transabdominal obstetric examinations, there is little benefit to the patient having a full bladder, and it has drawbacks (eg, false diagnosis of placenta previa or falsely elongated cervix). Urine in the bladder is useful when the area of the lower uterine segment is of interest as it can provide a helpful window for the evaluation. In transabdominal gynecologic examinations, the bladder does not have to be full; however, if the uterus and ovaries cannot be seen well, it may be necessary to have the patient fill her bladder to a comfortable capacity.

Transvaginal sonography is usually performed with an empty bladder.

REMOTE INTERPRETATION OF IMAGES — Both two-dimensional ultrasound images and three-dimensional datasets can be sent electronically for evaluation by experts [57-59] or to facilitate consultation between a patient or referring physician and a specialist [67]. The transfer of ultrasound images can be easily accomplished using the Digital Imaging and Communications in Medicine (DICOM) protocol, which is an international standard for exchange, storage and communication of digital medical images and other related digital data [67]. Provision of such consultative services to resource-limited or remote areas allows access to clinical expertise and potential improvements in diagnosis and management that may not have otherwise happened.

SOCIETY GUIDELINE LINKS — Links to society and government-sponsored guidelines from selected countries and regions around the world are provided separately. (See "Society guideline links: Ultrasound imaging in pregnancy".)

SUMMARY AND RECOMMENDATIONS

●Indications for obstetric and gynecologic ultrasound examination – The indications for obstetric ultrasound examination vary by trimester and are listed in the table (table 3) (see 'Indications' above). Gynecologic ultrasound examination has multiple uses for diagnosis of pelvic disorders and abnormalities (table 1). (See 'Indications' above.)

•The American College of Obstetricians and Gynecologists and others have recommended ultrasound examination for all pregnant patients. The timing and frequency depend on the indication for the examination. If only a single screening examination is performed, the optimal time is at 18 to 20 weeks of gestation. (See 'Obstetric sonography' above.)

●Standard components – Suggested components of a standard obstetric and gynecologic evaluation have been outlined by various groups. (See 'Scope' above and 'Standard examination' above.)

●Safety considerations – Diagnostic ultrasound examination used appropriately has no harmful effects (including fetal effects). Nevertheless, examinations should be performed only for valid medical reasons, for the shortest amount of time, and with the lowest level of acoustic energy that allows diagnostic evaluation. (See 'Safety' above.)

●Clinical use

•Sonography is an operator-dependent technology: A high level of proficiency can only be achieved by supervised experience with a large variety of normal and abnormal examinations. (See 'Personnel' above.)

•Several ultrasound modalities are available; the choice depends on the information needed. (See 'Ultrasound modalities' above.)