Version May 2024
INTRODUCTION — Delivery of the low birth weight (LBW) fetus is often necessitated by maternal and fetal indications, including nonreassuring fetal status, intractable preterm labor, intraamniotic infection, placental abruption, fetal growth restriction, hypertensive disorders of pregnancy, and others (see individual topic reviews on these subjects). The prognosis for survival and long-term outcomes of LBW neonates continues to improve and can be attributed, in part, to widespread administration of a course of antenatal corticosteroids to patients at risk for preterm birth, liberal use of cesarean birth for fetal indications, use of magnesium sulfate for neuroprotection, improvements in neonatal resuscitation and care (use of surfactant therapy), and delivery at facilities with resources for neonatal intensive care.
This topic will discuss intrapartum obstetric issues related to delivery of the LBW cephalic fetus. Other issues related to LBW are reviewed separately, including:
●(See "Preterm birth: Definitions of prematurity, epidemiology, and risk factors for infant mortality".)
●(See "Overview of short-term complications in preterm infants".)
●(See "Overview of the long-term complications of preterm birth".)
●(See "Periviable birth (limit of viability)".)
DEFINITION OF LOW BIRTH WEIGHT — Low birth weight (LBW) neonates are subgrouped according to the specific LBW category at the first weight determination after birth [1]:
●LBW – Less than 2500 grams
●Very low birth weight (VLBW) – Less than 1500 grams
●Extremely low birth weight (ELBW) – Less than 1000 grams
In this topic, we will use the term LBW to encompass both fetuses and neonates in all of these estimated weight and birth weight categories and only use the terms VLBW and ELBW when these subgroups are considered separately.
CHOICE OF FACILITY FOR BIRTH — Ideally, preterm births should occur in hospitals with nurseries able to provide an appropriate level of care. This is particularly important for very low birth weight (VLBW) infants and very preterm infants.
In a meta-analysis of studies of the association between nursery level at birth and neonatal mortality, VLBW infants born in hospitals with level I or II nurseries had higher mortality than those born in hospitals with level III nurseries (38 versus 23 percent; odds ratio [OR] 1.62, 95% CI 1.21-1.98) [2,3]. When analyzed by gestational age, infants ≤32 weeks of gestation born in hospitals with level I or II nurseries had higher mortality than those with level III nurseries (12 versus 7 percent; OR 1.42, 95% CI 1.06-1.88).
PREPARATION FOR DELIVERY — When time permits, the following interventions can reduce neonatal morbidity and mortality:
●Administration of a course of antenatal corticosteroids (ACS) to facilitate fetal lung maturation. The upper gestational age for ACS is typically the 34th week, but some providers extend this to the 36th week of gestation, particularly if the patients has not previously received ACS. (See "Antenatal corticosteroid therapy for reduction of neonatal respiratory morbidity and mortality from preterm delivery".)
●Administration of magnesium sulfate infusion to reduce the risk of neurological injury. The upper gestational age for administration is typically the 32nd week, but some providers extend use to the 34th week of gestation (See "Neuroprotective effects of in utero exposure to magnesium sulfate".)
●Administration of antibiotic prophylaxis to protect against early-onset neonatal group B streptococcal disease. Intrapartum antibiotic prophylaxis against GBS is administered to patients who have positive prenatal GBS cultures and those with other high-risk factors. (See "Prevention of early-onset group B streptococcal disease in neonates".)
●Neonatology consultation — Neonatal providers should have an opportunity to talk with and assess the mother before giving birth, if possible. This discussion should include neonatal care plans/procedures and potential neonatal mortality and morbidity. The extent of care should also be discussed, when appropriate. (See "Periviable birth (limit of viability)" and "Pediatric palliative care", section on 'Perinatal palliative care'.)
CHOOSING THE ROUTE OF BIRTH
Planned cesarean versus planned vaginal — We believe individuals with a LBW cephalic fetus should undergo a trial of labor in the absence of a standard indication for planned cesarean birth (eg, previous classical hysterotomy, placenta previa). Cesarean birth is associated with known risks for the mother and no proven benefits for the LBW neonate in cephalic presentation who is tolerating labor.
In contrast, vaginal birth of the LBW breech fetus (<32 weeks of gestation) is associated with an increased risk of perinatal mortality [4]. (See "Overview of breech presentation", section on 'Approach to management of preterm breech birth'.)
Perinatal issues and outcomes — The rationale for routine cesarean birth of the LBW cephalic fetus when preterm birth is necessary or inevitable is based upon the hypothesis that avoiding active labor and vaginal birth might improve neonatal survival by reducing hypoxic stress and risks of asphyxia and intraventricular hemorrhage (IVH) [5-7]. However, no high-quality evidence supports this hypothesis, and meta-analyses suggest that the route of birth is not a significant independent factor affecting perinatal mortality or neurodevelopment:
●In a meta-analysis of four randomized trials (n = 116 participants) comparing cesarean versus attempted vaginal birth for preterm singletons, there were no statistically significant differences in perinatal morbidity or mortality between groups [8]. However, the small number of pregnancies precluded making any conclusions about the optimal route of birth. The authors observed that one in six newborns did not deliver by the method to which they were allocated, in part due to precipitous births and to nonreassuring fetal heart rate patterns.
●In a meta-analysis of case-control and cohort studies that analyzed the association between cesarean birth and cerebral palsy, cesarean birth (planned or emergency) was not associated with a significant reduction in risk of cerebral palsy in preterm infants (odds ratio [OR] 0.81, 95% CI 0.47-1.40; six studies, n = 2416 births) [9]. The gestational age range for the preterm births varied widely among the included studies.
Most evidence from prospective and retrospective studies performed worldwide has not demonstrated significant improvement in perinatal morbidity or mortality for the cephalic fetus delivered by cesarean, particularly when the primary indication for operative birth was LBW and even at very early gestational ages (22 to 23 weeks) [10-26]. A longer-term study found that route of birth of periviable fetuses made no detectable difference in Bayley II scores at age two years [27]. Although a few studies have observed an increased risk of IVH or death among LBW newborns born vaginally [28-30], a consistent limitation of these studies is the marked differences in baseline characteristics between the cesarean and vaginal birth groups. Obstetric complications that favor birth by one route over the other are important confounders. Statistical adjustment for maternal and fetal confounding factors tends to eliminate route of birth as a significant factor in risk of either mortality or IVH [11]. (See "Germinal matrix and intraventricular hemorrhage (GMH-IVH) in the newborn: Risk factors, clinical features, screening, and diagnosis", section on 'Perinatal factors'.)
The LBW newborn may be preterm or small for gestational age (SGA); however, most studies have not distinguished between these two subgroups when evaluating outcome. There are limited observational data suggesting a survival advantage for some preterm SGA fetuses born by cesarean compared with those born vaginally [15,17,31]. This finding should be interpreted cautiously because of the limitations of observational data and because no studies have compared outcomes of prelabor cesarean birth versus planned vaginal birth. Some authors have hypothesized that labor has a deleterious effect on the preterm fetal brain and have suggested that cesarean birth may only reduce neonatal morbidity if it is performed before the fetus is exposed to labor. However, prelabor cesarean birth performed in an attempt to avoid exposing the fetus to the potentially deleterious effects of labor might lead to unnecessarily early delivery since expectant management of many pregnancy complications can lead to significant prolongation of pregnancy with further fetal growth and maturation. Furthermore, preterm labor itself is the most common cause or contributing factor to birth of LBW neonates and thus cannot be avoided in most cases.
Maternal issues and outcomes — Maternal outcome should also be considered and is related to several factors, including the route of birth, the gestational age at the time of birth, and the underlying etiology of the LBW birth. Cesarean birth of LBW fetuses, especially those that are very low birth weight (VLBW) or extremely low birth weight (ELBW), is more likely to require a vertical or classical hysterotomy incision. This places the mother at increased risk of serious complications, such as placenta accreta spectrum and uterine rupture, in future pregnancies. Even a low transverse incision may be associated with an increased risk of future uterine rupture when compared with the same incision at term (4 of 228 [1.8 percent] versus 36 of 9558 [0.4 percent] in one study [32]). In addition, a classical hysterotomy commits the patient to a cesarean birth by 37 weeks of gestation for all of their future pregnancies, which increases the risk of neonatal morbidity compared with ≥39 weeks, the usual gestational age for scheduled repeat cesarean in patients with a previous low transverse cesarean birth. Early gestational age at cesarean also increases maternal risk [33,34]. For example, in a meta-analysis of six studies and >45,000 pregnancies, severe maternal morbidity (eg, blood transfusion, and/or hysterectomy related to severe postpartum hemorrhage, intensive care unit admission, unexpected procedures, death) was more common after a cesarean birth before 26 weeks (17.6 percent versus 10.6 percent between 26 and 34 weeks; RR 1.65; 95 % CI 1.52–1.78) [33]. (See "Cesarean birth: Postoperative care, complications, and long-term sequelae", section on 'Complications'.)
Do forceps protect the head during vaginal birth? — Although compression of the fetal head increases cerebral venous pressure, which theoretically could promote IVH, there is no high-quality evidence that reducing maternal soft tissue compression of the fetal head during vaginal birth is an important factor in preventing IVH in fetuses without a bleeding diathesis.
For example, in a study that performed neonatal ultrasound on 288 neonates ≤2000 grams born from cephalic presentation either spontaneously or by low forceps, no association was observed between the type of vaginal birth and development of early periventricular hemorrhage or IVH [35]. Another study of 101 neonates weighing 500 to 1500 grams reported that factors believed to affect head compression, such as use of episiotomy and forceps, did not correlate with the frequency of IVH [36].
INTRAPARTUM ISSUES
Intrapartum fetal monitoring — Continuous intrapartum fetal heart rate monitoring of the LBW fetus is indicated if a nonreassuring fetal heart rate pattern would prompt intervention.
The immature fetal cardiovascular and nervous system of the preterm fetus results in fetal heart rate patterns that are normal for gestational age but slightly different from the typical patterns seen later in gestation. For example:
●Baseline fetal heart rate gradually decreases across gestation but remains within the normal range (110 to 160 beats/minute). Before 25 weeks, the mean fetal heart rate is 150 to 155 beats/minute, falling to 130 to 140 beats/minute at term [37-40].
●Before approximately 30 weeks of gestation, the frequency, amplitude, and duration of fetal heart rate accelerations are reduced. Nonhypoxic, nonacidotic preterm fetuses under 30 weeks may exhibit few accelerations, and the peak may be only 10 beats/minute above baseline and the acceleration may last only 10 seconds. After approximately 30 weeks, accelerations become more frequent in the healthy fetus; the peak should be at least 15 beats/minute and the acceleration should last at least 15 seconds [37,41].
●Healthy fetuses between 20 and 30 weeks of gestation may have occasional fetal heart rate decelerations unassociated with contractions [38].
●A reduction in baseline variability before approximately 28 weeks has been described but not clearly quantified [37,41].
●In labor units where fetal ST segment analysis (STAN) is used, this approach to fetal heart rate monitoring should be avoided prior to 36 weeks of gestation [42], as available data in preterm fetuses are not sufficient to determine whether the device is reliable.
These issues are described in more detail separately. (See "Intrapartum fetal heart rate monitoring: Overview" and "Umbilical cord blood acid-base analysis at delivery".)
Maternal analgesia/anesthesia — The choice of maternal analgesia/anesthesia for labor and/or birth should be guided primarily by maternal needs and the specific clinical scenario, given the absence of adequate data on the optimal approach for patients with an LBW fetus. Although a large prospective epidemiologic study (EPIPAGE) reported an increased risk of neonatal mortality in very preterm infants undergoing cesarean birth under spinal anesthesia compared with general anesthesia or epidural anesthesia, this study had several limitations (eg, confounding, nonstandardized anesthetic techniques) that preclude making a change in clinical practice [43].
The benefits and risks of various analgesic and anesthetic techniques are reviewed separately. (See "Pharmacologic management of pain during labor and delivery" and "Neuraxial analgesia for labor and delivery (including instrumental delivery)".)
Use of episiotomy, vacuum, and forceps
●Episiotomy – Prophylactic use of episiotomy for delivery of the LBW fetus does not appear to be associated with improved neonatal outcome [44-46]. Selective use of episiotomy is reasonable when clinically indicated, such as a mediolateral episiotomy in deliveries with a high risk of severe perineal laceration or the need to facilitate a more rapid birth in the setting of a possibly compromised fetus. (See "Approach to episiotomy".)
●Vacuum – Vacuum-assisted birth is contraindicated before 34 weeks of gestation because it may increase the risk of intraventricular hemorrhage (IVH) in the preterm LBW fetus. When an instrument-assisted birth is necessary before 34 weeks of gestation, forceps should be used instead of vacuum. (See "Assisted (operative) vaginal birth", section on 'Contraindications'.)
●Forceps – As discussed above, prophylactic use of low forceps is not indicated, as the bulk of observational data suggest that the risk of IVH is not reduced by use of low forceps compared with no forceps. (See 'Do forceps protect the head during vaginal birth?' above.)
Use of low forceps when clinically indicated seems reasonable, since the risk of harm to the LBW fetus does not appear to be higher than in term fetuses whose weight is appropriate for gestational age. Forceps that are smaller in some dimensions than standard forceps are available and are intended for use in a LBW or very low birth weight (VLBW) delivery. These forceps (eg, "baby" Elliot and "baby" Simpson forceps) have the same blade dimensions as standard forceps, but the handles and shafts are shorter and lighter. This design may allow gentler manipulation of the preterm fetus. We were unable to identify any published studies or manufacturer guidelines regarding the estimated fetal weights or gestational ages at which these instruments might be most useful. (See "Assisted (operative) vaginal birth", section on 'Minimum and maximum estimated fetal weight'.)
Optimal time for cord clamping — Delaying cord clamping by at least 30 to 60 seconds is recommended, when feasible, because it appears to improve the neonatal transition and several neonatal outcomes compared with early/immediate cord clamping. The physiologic changes and the evidence for benefits and risks of delayed cord clamping in preterm newborns are reviewed in detail separately. (See "Labor and delivery: Management of the normal third stage after vaginal birth", section on 'Early versus delayed cord clamping'.)
Techniques for atraumatic cesarean birth — Extremely low birth weight (ELBW) infants (preterm or growth-restricted) present many challenges at cesarean birth:
●The uterus is less distended than with a term fetus, and the lower uterine segment is less well-developed (thicker myometrium, smaller area). Thus, the hysterotomy incision is thicker and bloodier, and an adequate transverse incision to allow atraumatic fetal extraction may not be possible (discussed below). It is also easy to inadvertently lacerate the fetus if the deepest layers of the myometrium are not incised carefully, especially in the setting of preterm prelabor rupture of membranes. For these reasons, blunt entry into the amniotic cavity may be optimal.
A transplacental incision may be required in some cases, which may necessitate immediate cord clamping to reduce fetal bleeding and may increase maternal blood loss and the risk for fetal laceration.
●Since these fetuses are preterm and/or very small, their bones and soft tissues are more delicate and prone to injury. To avoid iatrogenic injury, the pressure applied to grasp and extract them should be much less than that used for a normal term fetus.
●If the fetal size is very small, the uterus will be significantly smaller and occupy less of the abdomen and pelvis than with larger, term fetuses. As a result, the mother's intestines, which are usually confined to the upper abdomen during a cesarean birth, may descend into the operative field and may need to be manually displaced with either retractors or packing.
To address all of the issues during the cesarean birth of the extremely preterm or growth-restricted fetus, the obstetrician should carefully consider the choice and size of skin and uterine incisions. Although a low transverse skin incision (Pfannenstiel) is still a reasonable choice, the surgeon should ensure that the skin incision length is adequate to provide the exposure requirements that they might encounter; in particular, the skin incision should not be made smaller to adjust for the smaller size of the fetus.
While some births can be accomplished safely via a low transverse uterine incision, the lack of development of the lower uterine segment often precludes this option. A vertical uterine incision may be a more prudent choice and guarantees adequate access to the fetus without the risks of head entrapment or injury to the uterine arteries. In one study, 50 percent of cesareans in singleton pregnancies ≤26 weeks of gestation were performed using a classical hysterotomy; the rate was lower in twin pregnancies, likely due to a more developed lower uterine segment [47]. As with any incision, the surgeon's judgment should ultimately dictate the choice.
For vertical incisions, some clinicians begin the incision in the lower uterine segment and cut toward the fundus. This minimizes the chances that bleeding from the incision site will obscure the next area to be incised and the incision never has to be extended blindly towards the bladder in a blood obscured field. Other clinicians begin the incision at its most cephalad point and cut toward the cervix. Whether one approach is better than the other has not been studied.
MANAGEMENT AT/NEAR THE LIMIT OF VIABILITY — The American College of Obstetricians and Gynecologists and the Society for Maternal-Fetal Medicine have published a consensus statement about clinical management of pregnancies with threatened or imminent delivery at 20+0 to 25+6 weeks of gestation; this guidance is summarized in the table (table 1) [48].
Pediatric organizations have published generally similar statements, which are reviewed separately. (See "Periviable birth (limit of viability)".)
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: Preterm labor and birth".)
SUMMARY AND RECOMMENDATIONS
●Medications to reduce morbidity of preterm birth – If time permits, administration of a course of antenatal corticosteroids and magnesium sulfate can decrease neonatal morbidity and mortality. (See 'Preparation for delivery' above.)
●Fetal heart rate monitoring – Continuous intrapartum fetal heart rate monitoring of the LBW fetus is indicated if a nonreassuring fetal heart rate pattern would prompt intervention. The immature fetal cardiovascular and nervous system of the preterm fetus results in fetal heart rate patterns that are normal for gestational age, but slightly different from the typical patterns seen later in gestation. (See 'Intrapartum fetal monitoring' above.)
●Site of delivery – Ideally, preterm births should occur in hospitals with nurseries able to provide an appropriate level of care and neonatology consultation. (See 'Choice of facility for birth' above and 'Preparation for delivery' above.)
●Route of delivery – For the cephalic low birth weight (LBW) fetus, we suggest avoiding planned (prophylactic) cesarean delivery (Grade 2C). A policy of planned cesarean delivery is associated with known risks to the mother, but the benefits to the LBW neonate in cephalic presentation are uncertain. Standard obstetric care aimed at avoiding a depressed neonate seems to be a better strategy than planned cesarean delivery for preventing asphyxia, intraventricular hemorrhage (IVH), and death in these fetuses. (See 'Planned cesarean versus planned vaginal' above.)
Delivery of the breech fetus is a separate issue. (See "Delivery of the singleton fetus in breech presentation".)
●Management of labor and delivery
•Analgesia/anesthesia – Choice of maternal analgesia/anesthesia should be guided primarily by maternal needs. (See 'Maternal analgesia/anesthesia' above.)
•Vaginal birth – Head compression by maternal soft tissues in the vertex LBW fetus is not a major determinant of IVH. For this reason, we suggest avoiding prophylactic episiotomy and low forceps delivery (Grade 2C). (See 'Do forceps protect the head during vaginal birth?' above and 'Use of episiotomy, vacuum, and forceps' above.)
-Vacuum-assisted birth – Vacuum-assisted birth is contraindicated before 34 weeks of gestation because it may increase the risk of IVH in the preterm LBW fetus. (See "Assisted (operative) vaginal birth", section on 'Contraindications'.)
•Cesarean birth – Obstetricians should carefully consider their choice and size of skin and uterine incisions. Although a low transverse skin incision (Pfannenstiel) is a reasonable choice, the skin incision should not be prorated smaller to adjust for the smaller size of the fetus. (See 'Techniques for atraumatic cesarean birth' above.)
•Cord clamping – We suggest optimal (also called "delayed") rather than early cord clamping, when feasible (Grade 2B). (See 'Optimal time for cord clamping' above.)
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