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Postterm infant

Postterm infant
Literature review current through: Jan 2024.
This topic last updated: Apr 10, 2023.

INTRODUCTION — Postterm infants are born at a gestational age (GA) greater than 42 weeks or 294 days from the first day of the last menstrual period. Postterm infants have higher rates of morbidity and mortality than term infants.

The epidemiology, clinical manifestations, neonatal management, and outcome of postterm infants will be reviewed here.

INCIDENCE — The incidence of postterm births varies with different patient populations and is dependent on a number of factors, such as the percentage of primigravid women, and local practice patterns regarding the rates of scheduled cesarean delivery and routine labor induction.

In the United States, postterm births have decreased from 11 percent of all births in 1990 to 5.5 percent of births in 2009 [1]. The overall incidence has decreased with the increased use of first trimester ultrasounds and the resultant improved dating of pregnancies [2].

In a population-based Norwegian study, the reported incidence of postterm deliveries was 7.6 percent from 1989 to 1999 [3].

In a study from a tertiary medical center in Israel, 3.2 percent of uncomplicated singleton births were born postterm (gestational age [GA] ≥42 weeks), 19.7 percent were born late term (GA between 41 and <42 weeks), and 77.1 were full-term (GA between 39 and <41 weeks) [4].

While there is no difference in incidence between males and females when dating is performed by last menstrual period, one large study concluded that when midtrimester ultrasound dating is employed, more females are born postterm [5], perhaps due to an underestimation of gestational age for female fetuses based on their smaller size [5].

In the United States, the decline in postterm births is the result of the widespread practice of inducing labor once pregnancies reach 41 weeks, a specific effort to avoid the delivery of postterm infants. (See "Postterm pregnancy", section on 'Management'.)

RISK FACTORS — The etiology of most postterm pregnancies is unknown. Risk factors for postterm births are discussed separately and include the following (see "Postterm pregnancy", section on 'Etiology'):

Primigravidity

Prior postterm pregnancy

Genetic predisposition as concordance for postterm pregnancy is higher in monozygotic than dizygotic twin mothers

Maternal obesity

Older maternal age

Male fetal sex

Specific conditions in the offspring associated with prolonged gestation include adrenal gland hypoplasia and congenital adrenal hyperplasia due to 21-hydroxylase deficiency [6]. Placental sulfatase deficiency also is a rare cause of postterm pregnancy.

It has been suggested that specific nutritional deficiencies, such as zinc, may predispose to altered pregnancy length, but systematic reviews of the available literature have not supported this finding [7]. Over the past twenty years there have been suggestions that there may be a specific genetic factor associated with postterm birth, although none has yet been identified [8-10].

CLINICAL MANIFESTATIONS — The clinical presentation of postterm infants is based primarily on fetal growth. In most cases, continued fetal growth results in higher birth weights in the postterm than term infant, with an increased likelihood of macrosomia. However, fetal growth restriction (FGR) occurs in some postterm infants, most likely caused by a poorly functioning placenta that is unable to provide adequate nutrition [11-13]. This results in the birth of a small for gestational age (SGA) infant, who usually appears malnourished. (See 'Fetal growth restriction' below.)

Macrosomia — Infant size and birth weight (BW) are affected by extended gestational length. Most postterm infants are larger than term infants [14-16], as both fetal BW and head circumference measurements continue to increase from 39 to 43 weeks gestation [17]. In a study from China, macrosomia (defined as a BW >4000 g) was more likely in postterm infants (gestational age [GA] ≥42 weeks) than in infants born at 40 weeks gestation (11 versus 9 percent, adjusted odds ratio [OR] 1.35, 95% CI 1.3-1.41) [16].

These infants appear normal at birth, apart from their large size. They often have accentuated physiologic desquamation of the skin, which peaks at approximately one week of age. Postterm macrosomic infants are at risk for birth injury due to prolonged labor, cephalopelvic disproportion, and shoulder dystocia [15]. (See "Large for gestational age (LGA) newborn", section on 'Birth injury' and 'Neonatal complications' below.)

Fetal growth restriction — Poor fetal growth is thought to be due to a degenerating or poorly functioning placenta that is unable to provide adequate nutrition and oxygenation. The risk of FGR increases as the pregnancy extends beyond 40 weeks gestation. As an example, in a population-based study of all deliveries that occurred in Sweden from 1987 to 1992, the incidence of FGR was 17, 20, 29, and 62 per 1000 pregnancies at gestational weeks 40, 41, 42, and ≥43, respectively [18]. Fetal growth restriction is associated with increased perinatal mortality in postterm pregnancies. (See 'Perinatal mortality' below.)

FGR in postterm pregnancy results in a long, thin, SGA malnourished infant with meconium staining, and dry and parchment-like peeling skin, referred to as dysmaturity syndrome [11,19,20]. In these infants, the skin appears loose, especially over the thighs and buttocks, and has prominent creases. Vernix caseosa is decreased or absent. Lanugo hair is sparse or absent, while scalp hair is increased. The nails typically are long. The infants generally have the appearance of increased alertness and a "wide-eyed" look.

PERINATAL MORTALITY — Perinatal mortality (ie, stillbirths plus early neonatal deaths) increases as the pregnancy extends beyond 40 weeks gestation, as demonstrated by the following [3,18,21,22]:

In the previously discussed population-based study from Sweden, logistic regression analysis demonstrated that intrauterine fetal death (stillbirth) rate rose with increasing gestational age (GA) beyond 40 weeks [18]. Although not statistically significant, there was a trend of increased neonatal mortality with increasing GA above 40 weeks. The presence of fetal growth restriction (FGR) increased the risk of both fetal and neonatal mortality as GA increased beyond 40 weeks. The analysis adjusted for maternal age, parity, maternal smoking, and fetal sex. A large population-based cohort study confirmed the significance of growth restriction as the major mortality risk in these infants [23].

In a population-based study from Norway, perinatal mortality was twofold greater in infants born at ≥42 weeks gestation compared with term infants (relative risk [RR] 2.0, 95% CI 0.9-4.6) [3].

In a retrospective review of singleton liveborn infants without congenital anomalies in California from 1999 to 2003, infants born between 41 and <43 weeks gestation had a higher mortality rate than those born between 38 and 40 weeks gestation (adjusted odds ratio [OR] 1.34, 95% CI 1.08-1.65) [24]. The neonatal death rates were 2.46, 2.7, 2.54, 3.66, and 3.28 per 10,000 live births for infants born between 38 and <39 weeks, 39 and <40 weeks, 40 and <41 weeks, 41 and <42 weeks, and 42 and <43 weeks, respectively.

Fetoplacental insufficiency resulting in poor fetal growth, asphyxia (with and without meconium), and intrauterine infection are thought to contribute to the excess perinatal deaths [18,25].

Other concomitant factors along with prolonged pregnancy that have been reported to be associated with increased perinatal mortality include:

Primigravidity – In one study, stillbirth risk did not differ by parity prior to 41 weeks gestation. However, the stillbirth risk was greater in nulliparous as compared with multiparous women at >42 weeks gestation (RR 2.9, 95% CI 1.06-8.19) [26].

Older maternal age – In a population-based study from Norway, the risk of postterm fetal death was greater in mothers aged 40 years compared with younger mothers <20 to 34 years of age, and 35 to 39 years of age (1.2 versus 0.3 and 0.5 percent) [27].

NEONATAL COMPLICATIONS — The risk of neonatal complications is greater in postterm compared with term infants. Among postterm infants, the types of complications tend to depend on how fetal growth has been affected.

Fetal macrosomia is associated with prolonged labor, cephalopelvic disproportion, and shoulder dystocia resulting in an increased risk of orthopedic (eg, clavicular fractures) or neurologic birth injury (eg, brachial plexus palsy). (See "Large for gestational age (LGA) newborn", section on 'Birth injury'.)

Postterm infants who had fetal (intrauterine) growth restriction (FGR) are at increased risk of perinatal depression or asphyxia, as the associated oligohydramnios may predispose to umbilical cord compression. Affected infants are at risk for neonatal complications typically seen in small for gestational age infants (SGA), such as hypoglycemia and polycythemia. (See "Infants with fetal (intrauterine) growth restriction".)

Both macrosomic and FGR postterm infants are at increased risk for perinatal asphyxia [11] and neonatal encephalopathy [28].

Postterm infants are more likely than term infants to have low Apgar scores, an indirect measure of perinatal asphyxia [3,29,30]. Meconium aspiration, congenital malformations, and persistent pulmonary hypertension are also more frequently observed in postterm infants [29,31]. (See "Meconium aspiration syndrome: Pathophysiology, clinical manifestations, and diagnosis" and "Persistent pulmonary hypertension of the newborn (PPHN): Clinical features and diagnosis".)

In one autopsy study, postterm infants were more likely than control term infants to have evidence of aspiration of amniotic fluid and/or meconium, which may have contributed to respiratory failure and death [32].

A large study that examined over 665,000 singleton births found an increased risk of oligohydramnios, meconium stained fluid, and birth trauma including facial nerve palsy in infants born postterm compared with term counterparts [33].

In the previously mentioned retrospective study from a tertiary Israeli hospital of 23,524 singleton births, multivariable analysis showed postterm birth (gestational age [GA] ≥42 weeks) was independently associated with an increased risk of neonatal intensive care admission, respiratory morbidity, culture-positive sepsis, and administration of antibiotic treatment compared with both infants born full-term (GA between 39 and <41 weeks) and late-term (GA between 41 and <42 weeks) [4]. Postterm infants also were more likely to have hypoglycemia compared with full-term but not late-term infants.

NEONATAL MANAGEMENT — Because the optimal intervention is prevention of postterm births, it has been suggested that induction of labor be routinely performed in mothers who are at 41 weeks gestation. (See "Postterm pregnancy", section on 'Our approach: Induction at 41+0 weeks'.)

Nevertheless, there are settings in which induction of labor at 41 weeks gestation is not an option. For these postterm deliveries, the neonatal management consists of screening and treating complications associated with prolonged pregnancy (eg, meconium aspiration, perinatal asphyxia, and persistent pulmonary hypertension), and providing routine newborn care.

Prior to delivery, an assessment of the need for neonatal resuscitation is made based on the gestational age (GA), anticipated birth weight (BW), presence of a congenital anomaly or labor complications, mode of delivery (eg, cesarean delivery), and maternal history. (See "Neonatal resuscitation in the delivery room", section on 'Anticipation of resuscitation need'.)

Immediately after delivery, routine neonatal care is provided that includes drying, clearing the airway of secretions if present, maintaining warmth, and a rapid assessment of the infant's clinical status based on vigor, including cry, heart rate and tone, as well as an examination to identify any major congenital anomaly.

The need for further intervention is based on this initial evaluation. If the infant does not require additional resuscitation, the infant should be returned to the mother for skin-to-skin care and initiation of breastfeeding right after birth. Infants should be fed as quickly as possible after delivery to avoid hypoglycemia. (See "Neonatal resuscitation in the delivery room", section on 'Resuscitation' and "Overview of the routine management of the healthy newborn infant", section on 'Delivery room care'.)

Further evaluation following transition from the delivery room includes a comprehensive examination to identify any evidence of birth trauma (eg, perinatal depression, brachial plexus injury, or clavicular fracture), congenital defects, or complications associated with prolonged pregnancy. (See 'Neonatal complications' above and "Persistent pulmonary hypertension of the newborn (PPHN): Clinical features and diagnosis" and "Meconium aspiration syndrome: Management and outcome" and "Perinatal asphyxia in term and late preterm infants" and "Clinical features, diagnosis, and treatment of neonatal encephalopathy".)

Laboratory screening for hypoglycemia and polycythemia should be performed in infants with evidence of poor fetal growth within the first one to two hours following delivery. (See "Pathogenesis, screening, and diagnosis of neonatal hypoglycemia", section on 'Screening' and "Neonatal polycythemia", section on 'Diagnosis' and 'Neonatal complications' above.)

If there are no significant complications that require further intervention, routine newborn care should be provided. (See "Overview of the routine management of the healthy newborn infant".)

LONG-TERM OUTCOME — Although data are limited on the long-term outcome of postterm infants, these patients may be at risk for neurodevelopmental complications as follows:

Cerebral palsy (CP) – Children born postterm appear to have an increased risk of CP compared with those who were born at term. This was illustrated in a population-based follow-up study from Norway that reported children delivered at a gestational age (GA) >42 weeks were more likely to develop CP than children born at 40 weeks gestation (relative risk [RR] 1.4, 95% CI 1.1-1.8) [34]. The prevalence of CP in children with a GA >42 weeks was 1.44 per 1000 patients. (See "Cerebral palsy: Epidemiology, etiology, and prevention".)

Epilepsy – In a cohort Danish study, it appears that prolonged gestation was a risk factor for early epilepsy that occurred within the first year of life [35]. However, there was no evidence of an association between postterm delivery and epilepsy beyond one year of age. A large Swedish study examining over 1 million births between 1983 and 1993 identified a higher incidence of childhood epilepsy (diagnosed up to age 20) among infants born postterm [36].

Developmental outcome – In an older cohort study from 1977, postterm infants had comparable motor scores on the Bayley Scales of Infant Development at eight months of age compared with controls, but cognitive scores were lower [37]. Such older studies may underestimate the impact of postterm birth, because they were done before routine ultrasound dating was in use. Many pregnancies may have been misclassified as postterm, resulting in an artifactual reduction in the estimated complication rates among postterm infants.

Behavioral problems – In a study of infants in the Netherlands, the risk of behavioral problems and attention deficit disorder was increased among postterm infants, compared with term counterparts [38].

In contrast, other follow-up studies report comparable outcomes for postterm and term infants.

At one and two years of age, the general intelligence quotient, physical milestones, and frequency of intercurrent illnesses were similar in postterm and term infants [13]. In a large retrospective study, the risk of general neurologic disability was similar in term and post term infants, although there was some increased risk of intellectual disability among the post term infants [39].

In another older study from 1977, Black children at seven years of age who were diagnosed with dysmaturity syndrome at birth had mental and physical development comparable to controls [40].

Analysis of linked data from Florida birth certificates and public school records from 1998 to 2013 of children born at 41 weeks (late term) compared with those born at 39 or 40 weeks resulted in a mixed results of cognitive and physical outcomes associated with late-term gestation [41]. Children born late term were more likely to have higher average test scores in mathematics and reading at ages 8 through 15 years of age and be physically impaired. Although these data are not directly applicable to children born postterm, it does provide indirect evidence of poorer physical outcome as pregnancy extends beyond term gestation (39 to 40 weeks).

Based on the available data, the long-term effects on the offspring of prolonged pregnancies remain unknown.

INFORMATION FOR PATIENTS — UpToDate offers two types of patient education materials, "The Basics" and "Beyond the Basics." The Basics patient education pieces are written in plain language, at the 5th to 6th grade reading level, and they answer the four or five key questions a patient might have about a given condition. These articles are best for patients who want a general overview and who prefer short, easy-to-read materials. Beyond the Basics patient education pieces are longer, more sophisticated, and more detailed. These articles are written at the 10th to 12th grade reading level and are best for patients who want in-depth information and are comfortable with some medical jargon.

Here are the patient education articles that are relevant to this topic. We encourage you to print or e-mail these topics to your patients. (You can also locate patient education articles on a variety of subjects by searching on "patient info" and the keyword(s) of interest.)

Basics topics (see "Patient education: When your baby is overdue (The Basics)")

Beyond the Basics topics (see "Patient education: Postterm pregnancy (Beyond the Basics)")

SUMMARY AND RECOMMENDATIONS — Postterm infants are born at a gestational age (GA) greater than 42 weeks or 294 days from the first day of the last menstrual period.

Risk factors for postterm delivery – Risk factors for postterm delivery include maternal factors (eg, primigravidity, prior delivery of a postterm infant, maternal obesity, and older maternal age), neonatal factors (eg, male sex and adrenal gland defects), and placental factors (eg, sulfatase deficiency). (See 'Risk factors' above.)

Clinical features of postterm infants – The clinical presentation of postterm infants is dependent on fetal growth.

In most cases, continued fetal growth leads to larger size infants for postterm compared with term deliveries and is associated with an increased risk of macrosomia (ie, birth weight [BW] >4500 g). (See 'Macrosomia' above.)

Fetal growth restriction (FGR) occurs in some postterm infants, resulting in the birth of a small for gestational age (SGA), malnourished infant with meconium staining and dry, peeling skin (referred to as dysmaturity syndrome). (See 'Fetal growth restriction' above.)

Perinatal mortality – Perinatal mortality rises as pregnancy extends beyond 40 weeks gestation. The presence of FGR increases the risk of perinatal mortality. (See 'Perinatal mortality' above.)

Neonatal complications – The risk of neonatal complications is greater in postterm compared with term infants. Macrosomic postterm infants are at risk for birth injuries that are primarily due to shoulder dystocia and include brachial plexus injury and clavicular fractures. In SGA postterm infants, complications include hypoglycemia and polycythemia. All postterm infants are at risk for perinatal asphyxia, meconium aspiration, and persistent pulmonary hypertension. (See 'Neonatal complications' above.)

Neonatal care – Neonatal management of postterm infants includes screening and treating complications associated with prolonged pregnancy and providing routine newborn care. (See 'Neonatal management' above.)

Long-term outcomes – The long-term effects of prolonged pregnancies remain unknown. Limited data suggest individuals born postterm are at risk for neurodevelopmental complications (eg, cerebral palsy [CP]). (See 'Long-term outcome' above.)

ACKNOWLEDGMENT — The UpToDate editorial staff acknowledges Lori A Sielski, MD, who contributed to an earlier version of this topic review.

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