20.1 Introduction

Duration of human pregnancy is known to be 280 days. In clinical practice expected date of delivery is forecasted by adding 280 days to date of last menstrual period. But a pregnancy is said to be “term” between 37 weeks and 42 weeks of gestation. There are several changes during this 5-week-long interval which determine the neonatal outcome especially the respiratory morbidity. Many observational studies have shown increased risks to both the fetus and the mother in continuing pregnancy beyond expected date of delivery [1, 2]. A large retrospective study evaluated fetal and neonatal mortality rates in 181,524 late-term and postterm pregnancies and found a significant increase in fetal mortality after 41 weeks of gestation compared with 40 weeks (odds ratio, 1.5, 1.8 and 2.9 at 41 weeks, 42 weeks and 43 weeks of gestation, respectively) [3]. A study by Caughey and Musci found an increase in the rate of meconium and intrauterine death for every week after 37 weeks’ gestation and a large increase beyond 41 weeks [4].

The American College of Obstetrician and Gynecologists (ACOG) has now endorsed a recommendation that the label “term” be replaced by designations early term (37-0/7 weeks through 38-6/7 weeks), full term (39-0/7 weeks through 40-6/7 weeks), late-term (41 0/7 weeks-through 41 6/7 weeks) and post term (42-0/7 weeks and beyond) [5]. Moreover, very often in clinical practice, there is imprecision in dating of pregnancy. This difficulty arises when there are mistaken dates or pregnancy occurs in lactational amenorrhoea or soon after withdrawal of oral contraceptive pills or when bleeding occurs in early part of pregnancy. Therefore, it is difficult to decide at what gestation the women should be delivered for good maternal and fetal outcomes.

In clinical practice the terms postterm, postdates, prolonged and postmature are often used interchangeably. Postmature term is used for the clinical syndrome in the infant showing features of pathologically prolonged pregnancy. The term postdates should not to be used as it is often not easy to date pregnancy in all the women accurately. Postterm or prolonged pregnancy should be the preferred term used to describe pregnancy extended beyond expected dates.

ACOG and the World Health Organization (WHO) have defined postterm pregnancy as that lasts 42 weeks (294 days) or more from the first day of the last menstrual period [6, 7].

20.2 Incidence and Risk Factors

The incidence of postterm pregnancy ranges from 4% to 19% [8]. The incidence varies with the method chosen to estimate period of gestation. Blondel et al. studied the incidence of postterm pregnancy based on the last menstrual period or sonographic evaluation at 16–18 weeks and found 6.4% incidence when gestation was calculated based on the last menstrual period, while it decreased to 1.9% when sonographic fetal biometry was used as a parameter for estimating gestation [9]. The time of sonographic evaluation also alters the incidence, e.g. Caughey et al. reported 2.7% incidence compared with 3.7% when sonographic assessment was done in first as compared to second trimester of pregnancy [10]. Therefore, using the menstrual dates alone to label an index pregnancy as postterm is often inaccurate. Both last menstrual dates and sonography should be used to label a patient as postterm.

The incidence also varies according to ethnicity of the mother. African American and Asian women are less likely to reach 41–42 weeks of gestation [11]. Primigravidas are at more risk of having a postterm pregnancy as compared to multigravidas. Recurrence of having a postterm pregnancy is also seen which points to genetic inheritance. Similarly, the daughters of women with history of giving birth to postterm babies are at a higher risk of having a postterm pregnancy. Laursen et al. found that maternal, but not paternal, genes influenced prolonged pregnancy [12]. Obesity (BMI > 25 kg/m²) is also an important risk factor for having a postterm pregnancy [13]. Other risk factors include prior postterm pregnancy and carrying a male fetus. Fetal disorders have also been associated with postterm pregnancies, such as fetal adrenal hypoplasia and anencephaly, since there is impaired hypothalamic-pituatary-adrenal axis in these fetuses leading to decreased corticotrophin releasing hormone (CRH) secretion which in turn delays onset of labour. Rare cause may be placental sulfatase deficiency which is of X-linked inheritance [14].

20.3 Diagnosis

The diagnosis of a postterm pregnancy should be based on both date of last menstrual period and an early sonographic estimation of gestation using crown rump length (CRL). A Cochrane database systemic review has shown that early sonographic evaluation leads to reduction in need for induction of labour in postterm pregnancy [15].

In case pregnancy is the result of in vitro fertilisation (IVF) or intracytoplasmic sperm injection (ICSI), the date of embryo transfer can be used for estimation of gestation. Other less reliable methods include date of positive serum/urinary human chorionic gonadotrophin (hCG), clinical uterine size measurement in early pregnancy, fundal height, perception of quickening or detection of fetal heart sound on Doppler.

20.4 Pathophysiology

Pathogenesis of prolonged pregnancy is not clearly understood. It appears that the mechanism responsible for parturition fails to be triggered. Corticotropin releasing hormone (CRH) produced by placenta has been related to the length of gestation [16]. In women who deliver postterm, the rate of rise of CRH is found to be slower [17].

As the gestation progresses beyond 37 weeks, a number of changes take place which play an important role in outcome of these women.

20.4.1 Placental Changes

With advancing gestation changes take place in placenta as well. Morphological changes that occur with placental senescence can be seen on ultrasound.

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  Grade 0—placenta is usually homogeneous in appearance, without echogenic densities, limited by a smooth chorionic plate. It is usually seen in early pregnancy.

  
  
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  Grade 1—the chorionic plate begins to acquire subtle undulations, and echogenic densities appear to be randomly dispensed throughout the placenta sparing basal layer.

  
  
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  Grade II—indentations in chorionic plate become more marked, echogenic densities appear in basal layer and comma-like densities seem to extend from chorionic plate into placental substance.

  
  
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  Grade III—the indentations in chorionic plate become more marked, giving appearance of cotyledons. There is increase in confluence of comma-shaped densities that become intercotyledon septations. Also, the central portion of cotyledons become echo-free (fall out areas), and large irregular densities in the form of acoustic shadows appear (Fig. 20.1).

Although grade 0 and I placentas are not seen in postterm pregnancies, both grade II and II are seen with equal frequency in postterm pregnancies. Therefore, placental grading alone is a poor marker for predicting postterm pregnancy. Grossly, the incidence of placental calcifications and infarction is increased as pregnancy advances beyond 38 weeks.

20.5 Fetal Complications

20.5.1 Postmaturity Syndrome

Shime et al. reported incidence of postmaturity syndrome in nearly 10% of pregnancies between 41 weeks and 43 weeks of gestation [18]. Postmaturity syndrome is an entity in which the fetus has unique appearance with wrinkled, patchy and peeling skin and thin long extremities classically described as “old man” looks (Fig. 20.2). Wrinkling is maximum in palms and soles. These features are suggestive of wasting and loss of subcutaneous fat. The infant is usually alert with open eyes and appears worried. The nails are long. This picture is not seen in all cases of postmaturity. The likelihood increases with associated oligohydramnios.

20.6 Maternal Complications