Once the gestational age is believed to be firmly established and the patient approaches 41 weeks of gestation, manage-ment options include induction of labor or antepartumfetal surveillance, which continue either until spontaneouslabor occurs or until approximately 42 weeks. In the United States, very few pregnancies are allowed to progress beyond 42 weeks and virtually none beyond 43 weeks. Factors thatinfluence management include the patient’s concerns, the assess-ment of fetal well-being, and the status of the patient’s cervix. Induction of labor is appropriate if the cervix is favorable and if the patient prefers such management. The risk of failed induction is low with a favorable cervix, and most authorities believe it is low enough to recommend delivery in light of the risk of increased fetal morbidity in the post-term period.
The data on preventing postterm pregnancy are con-troversial. Some studies show that sweeping the mem-branes may decrease postterm pregnancy; other studiesdiffer. Sweeping the membranes is a procedure by which the amniotic sac is gently detached from the uterine wall at the level of the cervix. This procedure is thought to re-lease prostaglandins, thus increasing cervical dilatation, making the cervix more favorable and sometimes leading to the onset of labor. Sweeping the membranes should not be performed until gestational age can be verified and the maturity of the fetus ensured.
If the gestational age is not well-established and the men-strual history and early ultrasound findings are not available, there is little additional information that can be used to deter-mine the best estimate of gestational age. Amniocentesis is notespecially helpful, because fetal lung maturity is rarely a question in the postterm evaluation. Once the best date is selected, a management plan similar to that for a postterm pregnancy with well-established gestational age is used.
If the cervix is not favorable, fetal well-being is monitored while awaiting spontaneous labor or ripening of the cervix, which makes induction appropriate.
Fetal evaluation has not been shown to decrease mortality in postterm pregnancy; however, it is also not associated with any negative outcomes. A variety of management schemes have been devised to monitor fetal well-being, though none has been shown to be superior. Thus, it is common practice to assess fetal well-being using several methods. Weekly monitoring of amniotic fluid volume is commonly used, as oligohydramnios at term is a sufficient indication for delivery. Nonstress tests (fetal heart rate monitoring), biophysical profiles (ultrasound evaluation of fetal fluid, movement, tone, and breathing), or oxytocin challenge tests may be used once or twice a week. Another option is the combination of amniotic fluid assessment and nonstress test, known as the modified biophysical profile. Doppler flow studies of the umbilical artery are not con-sidered useful. Dailyfetal movement counting is included in most manage-ment plans, with decreased perceived fetal movement being an indication for further, timely evaluation of fetal well-being. Results of these tests are most useful when consid-ered within the context of other conditions affecting the mother and the fetus. If fetal test results are nonreassuring, delivery is indicated.
The patient with an unfavorable cervix should be counseled about risks of induction of labor and the risks of continuing pregnancy with fetal evaluation to help in clin-ical decision making. Both management plans—inducing labor and continued fetal surveillance—are associated with low rates of maternal and fetal morbidity in the low-risk patient. Although there is no absolute time by which labor must be induced, most physicians believe that deliv-ery should occur between 41 and 42 completed weeks. Compared with expectant management, several studies of rou-tine induction at 41 weeks, using cervical ripening agents, have demonstrated lower cesarean delivery rates, lower perinatal mortality, decreased length of hospital stay, decreased hospital cost, and higher patient satisfaction. Several different agentsare now available for cervical ripening, including intra-cervical or intravaginal preparation of prostaglandin; Foley bulb placed through the cervix; and misoprostol. Oxytocin should ideally be initiated after the cervix is ripened.
Induction at 41 weeks is quickly becoming the preferred management.
Because of the risk of macrosomia-associated birth trauma, ultrasonographic estimation of fetal weight should be obtained before induction of labor in a postterm preg-nancy when macrosomia is suspected. If the estimated fetal weight is more than 5000 g in a woman who does not have diabetes or 4500 g in a woman with diabetes, cesarean de-livery should be considered. There is no accurate way of esti-mating fetal weight at term; ultrasonographic estimates havea calculation error up to 500 g late in pregnancy. Clinically determined estimated fetal weights by palpation of the patient’s abdomen and Leopold maneuvers are similarly inaccurate.
For patients who are postterm, special precautions are taken at the time of delivery to provide prompt eval-uation of the infant in the event of meconium passage. In a depressed infant, aggressive suctioning of the fetus with a laryngoscope decreases, but does not eliminate, the like-lihood of meconium aspiration syndrome
In a vigorous infant with meconium passage, laryngoscopy and aggressive suctioning have not been shown to decrease the risk of meconium aspiration syndrome, and are no longer recommended.
Similarly, routine amnioinfusion during labor with meco-nium passage is not recommended.