Case Studies

Case 8: Post-term pregnancy

A nulliparous woman presents for a routine prenatal visit at 41 weeks’ gestation. She is concerned that she has not yet gone into labor. Fetal wellbeing is reassuring.

• 1. How is post-term pregnancy defined? How often does it occur?

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  Correct answer: The mean duration of pregnancy is 40 weeks (280 days) dated from the first day of the last normal menstrual period (LMP). “Term” refers to the period from 37 weeks to 42 weeks of gestation. Post-term (prolonged) pregnancy is defined as a gestational age of ≥42.0 weeks (≥294 days) dated from the LMP. The incidence of post-term pregnancy depends on the population mix (including percentage of primigravid women, women with pregnancy complications, and incidence of preterm birth) and on local practice patterns (such as the rate of elective cesarean section delivery and routine labor induction). Overall, approximately 10% (range 3–14%) of low-risk singleton pregnancies continue beyond 42 weeks’ gestation and 4% (2–7%) will continue beyond 43 weeks in the absence of obstetric intervention. A policy of routine dating ultrasound examination at the first prenatal visit (as is commonly done in Europe but not the USA) will substantially decrease the incidence of post-term pregnancy.

• 2. What causes some pregnancies to continue post-term?

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  Correct answer: Risk factors for post-term pregnancy include primiparity and prior post-term pregnancy. In rare instances, post-term pregnancy may be associated with placental sulfatase deficiency or fetal anencephaly (in the absence of polyhydramnios). However, most post-term pregnancies have no known cause.

• 3. On further review, she is not certain of her LMP. Is this important and, if so, what criteria should be used to confirm gestational age?

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  Correct answer: Accurate pregnancy dating is critical to the diagnosis of post-term pregnancy. Gestational age can be regarded as accurate if two or more of the following criteria are present:

  Confirmation of gestational age

  Clinical criteria

  • ≥39.0 weeks have elapsed since the LMP in a woman with a regular menstrual cycle and no immediate antecedent use of oral contraceptives
  • Fetal heart tones have been documented for ≥20 weeks by non-electronic fetoscope or for ≥30 weeks by Doppler ultrasound

  Laboratory criteria

  • ≥36 weeks have elapsed since a positive (serum βhCG) pregnancy test
  • Ultrasound estimation of gestational age is considered accurate if it is based on crown–rump measurements obtained at 6–11 weeks’ gestation OR it is based on biparietal diameter measurements obtained before 20 weeks’ gestation

• 4. What are the risks to the fetus of post-term pregnancy?

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  Correct answer: The timely onset of labor and delivery is a critical determinant of perinatal outcome. Post-term pregnancy is associated with a significant increase in perinatal morbidity and mortality. When pregnancies exceed 42 weeks, perinatal mortality (stillbirths plus early neonatal deaths) increases to 4–7 per 1,000 deliveries compared with 2–3 per 1,000 deliveries at 40 weeks. Perinatal mortality at 43 weeks is fourfold higher than that at 40 weeks, and is five- to sevenfold higher at 44 weeks.

  Fetal morbidity is also increased after 42 weeks. Post-term infants are on average larger than term infants (around 2.5–10% of fetuses delivered after 42 weeks exceed 4,500 g in weight compared with 0.8–1% at 40 weeks) and, as such, are predisposed to complications associated with fetal macrosomia, including prolonged labor, cephalopelvic disproportion, and shoulder dystocia with resultant neurologic injury. Moreover, 20–40% of post-term fetuses suffer from “fetal dysmaturity (postmaturity) syndrome” with evidence of IUGR (intrauterine growth restriction) secondary to chronic uteroplacental insufficiency. Such pregnancies are at increased risk of umbilical cord compression, non-reassuring fetal testing, meconium aspiration, short-term neonatal complications (including hypoglycemia and seizures), and long-term neurologic sequelae.

  At 1 and 2 years of age, the general intelligence quotient (IQ), physical milestones, and frequency of intercurrent illnesses is the same for normal term infants and those from uncomplicated prolonged pregnancies.

• 5. Does post-term pregnancy pose any increased risk to the mother?

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  Correct answer: Although much of the focus of post-term pregnancy has been on the fetus, studies have shown that post-term pregnancy also poses significant risk to the mother (see below).

  Maternal risks of post-term pregnancy

  Maternal complication	Incidence (%)
  Labor dystocia	9–12 (versus 2–7 at term)
  Cesarean section delivery	1.5- to 2-fold increase
  Severe perineal trauma	3.3 (versus 2.6 at term)
  Postpartum hemorrhage	10 (versus 8 at term)

• 6. What are the current recommendations for the management of post-term pregnancy?

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  Correct answer: In the past, the American Congress of Obstetricians and Gynecologists (ACOG) has recommended induction of labor for well-dated low-risk pregnancy sometime during week 43 of gestation. More recently, however, the ACOG has been unwilling to describe any specific upper limit of gestational age for expectantly managed pregnancies. The decision of whether or not to proceed with induction of labor should be made together with the patient, and should take into account such factors as precise gestational age, fetal wellbeing, amniotic fluid volume, and the degree of dilation and effacement of the cervix. Most practitioners now routinely offer induction of labor at 41 weeks’ gestation and few will allow pregnancy to continue beyond 42 weeks.

• 7. After weighing the risks and benefits of induction of labor, the patient and her husband decide that they would like to go home and await the spontaneous onset of labor. How should this patient be followed?

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  Correct answer: Given that the decision has been made to continue expectant management of this low-risk pregnancy at 41 weeks’ gestation, antepartum fetal surveillance should probably be initiated. Despite universal acceptance of antepartum fetal testing in post-term pregnancies (>42 weeks), there is insufficient evidence to show that it significantly improves perinatal outcome or that there is any benefit to routine fetal testing at 40–42 weeks’ gestation. Moreover, no single antenatal surveillance protocol for monitoring fetal wellbeing in post-term pregnancy appears to be superior to any other. Most authorities recommend twice-weekly fetal non-stress testing, biophysical profile, and/or ultrasound with amniotic fluid estimation. Delivery should be effected immediately if there is evidence of fetal compromise or oligohydramnios.