Clinical questions

1. What are risk factors for post‐term pregnancy?

When evaluating a patient’s risk for a post‐term pregnancy, it is necessary to confirm that pregnancy dating is indeed correct. The most common way of dating a pregnancy, last menstrual period, assumes that a woman has regular cycles with a 14 day follicular phase. With irregular cycles or a shorter/longer follicular phase, dating by ultrasound, not last menstrual period, is more reliable and decreases the risk of labor induction for post‐term pregnancy [4, 5]. Literature has shown that the use of first trimester ultrasound screening is effective in reducing post‐term labor induction rates [6].

Large epidemiologic studies have evaluated additional risk factors for post‐term pregnancy in well‐dated gestations. Using data from the Danish Birth Cohort, 53 392 participants with live‐born singleton deliveries between 1998 and 2001 were interviewed at 12 and 30 weeks’ gestation, and 6 and 18 months after delivery. In this study, increased BMI (25–29 kg m⁻²) and nulliparity were identified as risk factors for post‐term pregnancy, with adjusted odds ratios (AORs) of 1.24 (95% CI 1.15–1.34) and 1.35 (95% CI 1.27–1.44), respectively [7]. The risk of being post‐term increased by 37% with a BMI at 30–34 kg m⁻²; at a BMI of 35 kg m⁻² or more the adjusted odds ratio (OR) was 1.52 (95% CI 1.28–1.82) [7]. A population‐based observational study used data from the Cardiff Birth Survey to evaluate pregnancy outcomes according to BMI in otherwise uncomplicated singleton pregnancies [8]. In this study, women with a BMI > 30 kg m⁻² were at an increased risk of postdates pregnancy and more likely to require induction of labor because of prolonged pregnancy. A retrospective study of 9336 term births in California evaluated the association between pre‐pregnancy BMI and the length of pregnancy and found that higher pre‐pregnancy BMI was associated with higher risk of progressing past 40 weeks [9]. In this study, 28.5% of obese women progressed to >41 weeks’ gestation vs. 18.3% and 21.9% of underweight and normal weight women, respectively (p < 0.001). Interestingly, in logistic regression analysis with gestational age of >41 weeks as the outcome and BMI as a continuous predictor, an increase of 1 BMI unit was associated with an adjusted odds ratio of post‐term pregnancy of 1.29 (95% CI, 1.21–1.38). The above studies suggest a potential influence of hormonal factors in the pathway to parturition.

A retrospective cohort study of term singleton pregnancies at a managed care organization found obesity, nulliparity, and maternal age 30–39 years and 40 years and older were also risk factors for prolonged and post‐term pregnancy [10]. The mechanisms for these associations remain unclear.

Previous post‐term pregnancy is another risk factor for a repeat post‐term pregnancy. Using data from the Danish medical birth registry on a 5% random sample of women with two or more pregnancies between 1980 and 1992, the recurrence risk for post‐term pregnancy was 19.8% [11]. The study also found a tendency for more post‐term deliveries the longer the interpregnancy interval, suggesting both genetic and environmental predispositions to post‐term delivery [11]. Studies have also found that women had a reduced risk of recurrent post‐term pregnancy if they changed partners between pregnancies, suggesting that post‐term delivery may be in part determined by paternal genes [12].

Rare conditions associated with prolonged pregnancy include anencephaly, placental sulfatase insufficiency, absence of the fetal pituitary, and fetal adrenal hypoplasia in the human fetus [13, 14]. Male sex has also been identified as a risk factor for post‐term pregnancy [3]. One common finding among these conditions is the lack the high concentrations of estrogen that are usually seen in normal pregnancy.

1. 2. What are the risks of post‐term pregnancy to mother and fetus?

Maternal and fetal risks increase in post‐term pregnancies [15]. A study using a 10 year cohort of Norwegian births found an increased risk of obstetric trauma and labor dysfunction, related to both large for gestational age size and post‐term birth when compared to term births [16]. A cross‐sectional study using records from the Danish Medical Birth Registry evaluated women who gave birth to a singleton liveborn infant at >42 weeks of gestation and compared them to a random sample of women delivered spontaneously at term. The study found an overall maternal complication rate of 30%, including postpartum hemorrhage (AOR 1.37 [95% CI 1.28–1.46]), cephalopelvic disproportion (AOR 1.91 [95% CI 1.77–2.07]), cervical rupture (AOR 1.45 [95% CI 1.26–1.67]), dystocia (AOR 1.71 [95% CI 1.30–2.25]), intrapartum fetal demise (AOR 3.14 [95% CI 1.11–8.90]), cesarean delivery (AOR 1.58 [95% CI 1.51–1.66]), and infection (AOR 1.21 [95% CI 1.03–1.41]) [17]. In this study, the risk of maternal injury was not related to neonatal birthweight. An increased risk of cesarean has not been uniformly reported in all studies. A study comparing induction of labor vs. serial monitoring in post‐term pregnancy found a lower risk of cesarean delivery in induced post‐term patients (21.2% vs. 24.5%, p = 0.03). Interestingly, the lower risk of cesarean delivery was mostly due to a lower number of cesareans performed for fetal heart rate abnormalities in the women induced and there was no difference in the rate of cesareans for failure to progress or failed induction between the groups [18]. A recent retrospective cohort study of low‐risk term women evaluating the risk of perinatal complications by gestational age found that delivery at 41 weeks was associated with a higher overall febrile morbidity and cesarean delivery [19].

Fetal risks of post‐term pregnancy include macrosomia, intrauterine growth restriction, oligohydramnios and intrauterine fetal demise. An earlier study using records from over 370 000 reported births between 1987 and 1989 in New York City evaluated the residual prospective risk of stillbirth as a function of gestational age [20]. By 40 weeks the risk of stillbirth was 1 in 475, rising progressively to one in 375 at 43 weeks [20]. A study from a 10 year cohort of Norwegian births found that in post‐term births, risk factors for perinatal mortality included small for gestational age (adjusted relative risk 5.68; 95% CI 4.37, 7.38) and maternal age >35 years (adjusted relative risk 1.88; 95% CI 1.22, 2.89) [16]. Data from the Danish Medical Birth Registry found an increased risk of dystocia (AOR 1.71 [95% CI 1.30–2.25]) and intrapartum fetal demise (AOR 3.14 [95% CI 1.11–8.90]) in women who gave birth at >42 weeks of gestation [17]. A recent retrospective study of women delivered beyond 37 weeks gestation from 1992 to 2002 at a single community hospital found an increased risk of complications as gestational age advanced, with a significant increased rate of intrauterine fetal death beyond 41 weeks gestation [21]. The risk for intrauterine fetal demise was more than 2.5 times greater between 41 and 42 weeks of gestation as compared with before 40 weeks of gestation [21]. In another study comparing outcomes in term pregnancies by week of gestation, delivery at 41 weeks had a higher risk of birthweight greater than 4500 g (AOR 3.57 [95% CI 3.45–3.69]), neonatal injury (AOR 1.27 [95% CI 1.17–1.37]) and meconium aspiration (AOR 2.12 [95% CI 1.91–2.35]) when compared to deliveries at 39 weeks gestation [19]. See Table 41.1 for summary of risks.

1. 3. What is the optimum timing for delivery in a post‐term pregnancy?

There is persistent controversy about the optimal timing for delivery in pregnancy. Prior to the introduction of routine ultrasound for accurate dating, there was significant inaccuracy for assigning an estimated due date. This is due to the challenges in estimating conception timing when there are highly variable times of ovulation within variable menstrual cycles. Because of the inaccuracy in estimating true dates, pregnancies were often allowed to continue beyond 42 weeks as long as fetal surveillance was reassuring. This was acceptable since many of those pregnancies were not truly post‐dates. In a study of over 44 000 women, the impact of different methods of dating a pregnancy on the incidence of pregnancies lasting beyond 42 weeks was assessed [22]. The rate of delivery beyond 42 weeks was 6.4% using only menstrual dating versus 1.9% when based on ultrasound only. First trimester ultrasound appears to have a greater effect on accurately dating a pregnancy than only second trimester dating [6]. Overall the risk for having an induction of labor for a post‐term pregnancy is estimated to be reduced by 41% with the use of early ultrasound dating [23].

There is an increased risk for maternal complications as pregnancy progresses beyond 40 weeks including cesarean delivery, operative vaginal delivery, third and fourth degree lacerations, postpartum hemorrhage and febrile morbidity [17, 21, 24, 25]. There are also reported increased fetal and infant risks with advancing gestation such as stillbirth and perinatal death, abnormal acid‐base status, birth trauma, sepsis, intracranial hemorrhage, meconium aspiration syndrome and respiratory distress [17, 21, 25–30]. Therefore, it is reasonable to establish a management plan that balances those increasing risks of prolonging a pregnancy with the risks for intervention through induction of labor. Research to assess the optimum timing of delivery has been challenging due to the difficulty in selecting the appropriate methodology to use for studies as well as the selection of appropriate outcomes.

A number of early studies that tried to assess the relative risks versus benefits of delivery interventions compared outcomes of women delivered with induction of labor versus spontaneous labor. Those studies relatively consistently demonstrated an increased risk for cesarean delivery with induction of labor [31–37]. Not surprisingly, there were greater maternal and infant complications in the induction groups. The difficulty with these earlier studies was inherent in their study designs. It is not fair to compare spontaneous labors to induced labors, since spontaneous labor would be expected to have an easier course with fewer complications. Induction of labor is a management intervention whereas spontaneous labor is an event. The better study design would be a comparison of labor induction versus expectant management at each week of gestation past 40 weeks. There are a number of trials that have employed this methodology. Gülmezoglu et al. performed a meta‐analysis of trails comparing labor induction to expectant management at 41–42 weeks of gestation. Induction of labor at or beyond 41 completed weeks was associated with fewer perinatal deaths (Risk Ratio (RR) 0.3 95% CI 0.09, 0.99), with no evidence of a significant difference in the cesarean section rate for women in the induction group [38]. In a more recent meta‐analysis of randomized‐controlled trials comparing induction of labor versus expectant management for post‐dates that included additional studies (total of 19), there was a 15% reduction in the cesarean delivery rate with an OR of 0.85 (95% CI: 0.76, 0.95) [39]. There were no differences in the risks for cesarean delivery for fetal distress, operative vaginal delivery, postpartum hemorrhage, and several neonatal outcome measures. The risk for perinatal death was 63% reduced with induction of labor although this was only borderline significant with an OR of 0.37 (95% CI of 0.14, 1.00).

Based on the available information, it appears that induction of labor once a person reaches 41 weeks confers a small advantage for a modestly lower cesarean rate without causing either maternal or fetal harm. There may be an advantage for decreasing stillbirths, but the absolute risk is very low. As a variety of societies have suggested, it is reasonable to consider induction of labor once a pregnancy reaches 41 0/7 weeks, but women can also elect expectant management with fetal surveillance at least twice weekly until 42 0/7 weeks, after which delivery should be encouraged [15, 40, 41] (Figure 41.1).

Figure 41.1 A suggested management algorithm for prolonged pregnancies.

For women with prolonged pregnancies who enter labor spontaneously, there are no specific recommendations for changing labor management from standard obstetric procedures. For those who undergo an induction of labor, cervical ripening (prostaglandin agents or mechanical devices such as a balloon catheter) should be considered for an unfavorable cervix (Bishop’s score of ≤6), with oxytocin reserved for those with a favorable cervix [42].

In summary, fetal surveillance allows continuation of pregnancy beyond 40 weeks, but the ability of the various testing strategies to predict fetal compromise and prevent fetal death are limited for post‐term pregnancies. Given the increase in perinatal morbidity and mortality that accelerates at 42 weeks and the increase in maternal complications as pregnancy progresses, it is reasonable to deliver otherwise uncomplicated pregnancies with normal fetal testing and normal amniotic fluid volumes (AFVs) between 41 0/7 and 41 6/7 weeks of gestation. Pregnancies can be allowed to go beyond 42 0/7 weeks, but only after thorough counseling of the mother with informed consent about the increasing risks and decreasing benefits for pregnancy continuation.

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