(1)
Department of Family Medicine, University of California, Riverside, Riverside, CA, USA
Key Points
1.
Term pregnancy is defined as 37–42 weeks’ gestation.
2.
Accurate pregnancy dating is critical to assessment and management of postdates pregnancy.
3.
Timing of delivery should be prior to 42 weeks’ gestation and earlier if antenatal testing is nonreassuring.
Background
When a firm estimated date of delivery (EDD) is established early in pregnancy, providers can anticipate that most pregnancies will result in spontaneous delivery at term. Term in this setting is defined as 38–42 weeks’ gestation. Under some circumstances, however, pregnancy may continue beyond 42 weeks, requiring assessment and management as a postdates pregnancy. Although the exact number of pregnancies that continue beyond term is not well established (3–12 %), approximately 10 % of all pregnancies will result in induction of labor (although not all for postdates pregnancy). The cause of prolonged gestation is not well understood but patients with one postterm delivery have a 50 % likelihood of subsequent postterm delivery.
A significant first step in identifying postdates pregnancies is confirmation of gestational dating. As noted in Chap. 3, a variety of measurements may be used to establish the EDD, including the last menstrual period (LMP) and obstetrical ultrasound (US). Confirmation of the EDD is critical to appropriate management of postdates pregnancy. For this reason, all such data should be reviewed carefully and confirmed.
Although pregnancy is not considered postdates until 42 weeks of gestation, planning for management should begin at or near the EDD. Careful fetal monitoring and management for delivery is critical as postdates pregnancy is associated with an increased risk for operative delivery, macrosomia, shoulder dystocia, meconium aspiration, and fetal mortality (twice baseline at 42 weeks, six times baseline by 44 weeks).
Diagnosis
Approximately one-half of all postdates pregnancies are caused by inaccurate gestational dating. For this reason, confirmation of the appropriate gestational age is critical.
History
The patient’s menstrual history should be reviewed, including the timing and normality of the LMP. Under a variety of conditions, the episode of bleeding considered to be the LMP may be inaccurate. Oligomenorrhea, prior use of contraception such as oral contraception or medroxyprogesterone, and pregnancy-related first-trimester bleeding may all alter the accuracy of menstrual history. First-trimester bleeding per vagina is very common and such bleeding may be interpreted as menstrual bleeding when, in fact, it was not. Early pregnancy bleeding is reviewed in Chap. 9.
The date of the first positive pregnancy test may be helpful in narrowing the possible dates of pregnancy. A review of the prenatal record should include obstetrical US results, if available, fundal height measurements, fetal quickening, and first noted fetal heart tones by US (4–6 weeks), handheld Doppler (10–12 weeks), or fetoscope (18–20 weeks). A pelvic examination with bimanual assessment of uterine size early in pregnancy may also provide confirmatory support for EDD.
Prior obstetrical history should be reviewed as a past history of postdates delivery is associated with an increased risk of subsequent postdates delivery.
Physical Examination
Primary confirmation of postdates pregnancy is generally provided by a careful history. Physical examination is generally supplementary at term and should not alter an otherwise well-established EDD.
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