The reported incidence of post-term pregnancy is 3–15%, with an average of approximately 10%. This relatively wide range reflects, in part, the difficulty inherent in accurately defining pregnancy dates. The most common criterion used to establish gestational age is the menstrual history. However, numerous reports suggest that menstrual history alone is unreliable. Warsof reported uncertain menstrual histories in 45% of 4000 pregnancies in a 5-year study [3]. Using the LMP as the sole dating criterion, 11% of pregnancies were considered post term, compared to 6% when pregnancy dates were established by early ultrasound. Among 15,241 pregnancies, Tunon reported a post-term incidence of 10% using menstrual dating criteria compared to 4% using sonographic criteria [4]. Mongelli et al. studied 34,249 computer files of singleton pregnancies with “certain” menstrual dates and sonographic biometry [5]. Compared to menstrual history alone, sonographic estimation of dates yielded a 70% reduction in post-term pregnancies. A systematic review of nine trials by Neilson reported similar findings. Routine ultrasound resulted in reduced rates of labor induction for post-term pregnancy [6]. These reports are consistent with the experience of most obstetricians that the menstrual history is unreliable as the sole criterion for pregnancy dating.

Sonographic assessment of gestational age is most precise when performed early in pregnancy. A crown–rump length between 5 and 12 weeks is accurate to within ±5 days (±2 SD). The fetal biparietal diameter is accurate to ±8 days at 12–20 weeks, ±14 days at 20–30 weeks, and ±21 days beyond 30 weeks. The femur length is accurate to ±7 days at 12–20 weeks, ±11 days at 20–30 weeks, and ±16 days beyond 30 weeks. During the late second trimester, the best estimate is obtained from the average of the biparietal diameter, head circumference, abdominal circumference, and femur length. If the estimated gestational age (EGA) by these measurements differs from that derived by the LMP by more than 2 weeks in the second trimester, consideration should be given to recalculating the dates. During the third trimester, sonographic gestational age assessment is of limited use in dating a pregnancy.

Macrosomia

Macrosomia, defined as a birthweight in excess of 4500 g, has been reported in 2.8–5.4% of post-term infants compared to 0.8% of term infants. At any gestational age, fetal macrosomia is associated with increased risks of shoulder dystocia (14% vs 0.3%), birth trauma (11% vs 2%), and cesarean delivery (35% vs 17%).

Oligohydramnios

Oligohydramnios is observed with increased frequency in the post-term gestation. Phelan reported that the amniotic fluid volume, as estimated by the Amniotic Fluid Index, increased steadily in the first half of pregnancy, reaching a plateau of approximately 12 cm during the third trimester [7]. Between 40 and 42 weeks, it declined by as much as 30%. Using dye dilution, Beischer et al. demonstrated that the amniotic fluid volume declined by 30% after 42 weeks and by 50% after 43 weeks. The increased morbidity associated with oligohydramnios is well documented. Crowley et al. [9] and Leveno et al. [10] observed increased incidences of meconium-stained amniotic fluid and cesarean section for fetal distress in association with diminished amniotic fluid volume. Other associations include lower Apgar scores and umbilical artery pH values, and increased rates of fetal distress, cesarean delivery, meconium aspiration syndrome, and umbilical cord compression leading to variable FHR decelerations.

Meconium

Numerous reports have described an increased frequency of meconium passage in post-term pregnancies. The incidence of 25–30% represents a twofold increase over that observed at term. In the presence of diminished amniotic fluid volume, the incidence may be as high as 71%. However, meconium passage alone is not a reliable indicator of intrauterine fetal compromise. In many cases, meconium passage may simply reflect a maturing fetal vagal system. Alternatively, it may reflect stimulation of the vagal system by relatively mild degrees of fetal stress. Even when meconium passage is not secondary to fetal stress or distress, it poses the risk of meconium aspiration syndrome (MAS). The risk is compounded by diminished amniotic fluid volume, resulting in thick, undiluted meconium that is more likely to obstruct the airways. The incidence of MAS in the presence of meconium-stained amniotic fluid is approximately 2–4.5%. Reduction in MAS has been reported following adequate suctioning of the oropharynx prior to the first breath. MAS is encountered most often in high-risk gestations exhibiting abnormal FHR findings, but has been described in the absence of observed fetal distress.

Postmaturity

Approximately 10–20% of post-term fetuses exhibit clinical signs of the “postmaturity” or “dysmaturity” syndrome, including reduced subcutaneous tissue, dry, wrinkled, peeling skin, and meconium staining. Other observations include hypothermia, hypoglycemia, polycythemia, and hyperviscosity. These findings, present in 3% of term infants, are thought to reflect subacute placental insufficiency leading to nutritional deprivation, fetal wasting, decreased fat and glycogen stores, and chronic hypoxemia with compensatory hematopoiesis. Although the late consequences of this disorder are incompletely understood, infants have been reported to regain weight rapidly and exhibit few long-term neurologic sequelae.

In an accurately dated pregnancy, the evidence suggests that a fetus stands to gain nothing by remaining in utero