Postdate Pregnancy

INTRODUCTION

Postterm pregnancy includes pregnancies that last longer than 42 weeks. Postdate pregnancies last longer than the established or estimated date of confinement, also known as the due date in layman's terms(ie, 40 weeks). This chapter will address management of postdate pregnancies.

The reported frequency of postterm pregnancy is approximately 3-12%. Assuming that the incidence of postdate pregnancy may be slightly higher than that of postterm pregnancy is reasonable.

An obvious cause of both postterm and postdate pregnancy is inaccurate dating criteria. For a patient who presents late in pregnancy and is unsure of her last menstrual period, ultrasound dating can be inaccurate.

When using ultrasound criteria for the dating of a pregnancy, it is necessary to understand the margin of error reported at various times during each trimester. One must consider a calculated gestational age by composite biometry from an ultrasound as an estimate, and take into account the range of possibilities. For example, Crown Rump Length is +/- 3-5 days, ultrasound done at 12-20 weeks is +/- 1 week, at 20-30 weeks is +/- 2 weeks and after 30 weeks is +/- 3 weeks. So a pregnancy that is 35 weeks by a 31 week ultrasound could really be anywhere from 32 weeks to 38 weeks (35 weeks +/- 3 weeks). The importance of determining by what method a pregnancy is dated cannot be emphasized enough, as this may have significant consequences if one delivers a "term" pregnancy that is not or observes a "term" pregnancy that is very postterm.

A way to more accurately date a pregnancy when composite biometry is not consistent in all of the parameters (Biparietal diameter, Head circumference, Abdominal circumference and Femur length), is to look at the transcerebellar diameter. The diameter in millimeters corresponds to weeks of gestation up to 24 weeks, and thereafter charts are available to calculate gestational age. If one is unable to get the transcerebellar diameter, the "gold standard" for dating in the second trimester is the biparietal diameter (BPD).

TIMING OF DELIVERY

The first decision that must be made when managing a postdates pregnancy is whether or not to deliver. In certain cases; nonreassuring surveillance, oligohydramnios, growth restriction, and certain maternal diseases, the decision is straight forward. In these high risk situations, the time at which the risks of remaining pregnant begin to outweigh the risks of delivery may come at an earlier gestational age. However, frequently there are several options to consider when determining a course of action in the low risk pregnancy. The certainty of gestational age, the cervical exam, estimated fetal weight and past obstetrical history all need to be considered when mapping a course of action. It is also wise to involve the patient in this discussion, as her feelings and understanding of the situation are important as well.

Perinatal morbidity and mortality do not increase appreciably between 40-41 weeks. However, there are several complications associated with longer gestations. With the exception of uteroplacental insufficiency and subsequent growth restriction, postterm pregnancies tend to be larger than term pregnancies. The risk for macrosomia, shoulder dystocia and cephalopelvic disproportion increase in postterm pregnancies. In addition, the risk for perinatal mortality increases in the postterm pregnancy (Mannino, 1988). These complications support the idea that well dated pregnancies ought not to progress beyond 42 weeks, but the question still remains about how one should manage a pregnancy between 41-42 weeks.

Delivery versus expectant management of low-risk pregnancies at 41 weeks has been addressed recently in the literature. The main argument against a policy of routine induction of labor at 41-42 weeks has been that induction will increase the rate of cesarean section without decreasing maternal and/or neonatal morbidity. Some of the studies that failed to show a reduction in fetal/neonatal morbidity were diluted by poorly dated pregnancies that weren't necessarily postterm. In addition, the potential for increasing the risk for cesarean section with a failed induction is far less likely in the era of safe and effective cervical ripening agents.

In a retrospective review of 18,055 singleton pregnancies, Yeast et al found no differences in cesarean section rate in women entering labor spontaneously and those who had labor induced. Herabutya et al, the National Institute of Child Health and Human Development, and the Canadian Multicenter Postterm Pregnancy Trial have completed 3 recent prospective randomized studies. No increase in the rate of cesarean section was found in patients who were randomized to routine induction of labor. In fact, more cesarean sections were performed in the noninduction groups, and the most frequent indication was fetal distress. The neonatal outcomes were similar in both the routine induction and noninduction groups. All 3 trials concluded that the incidence of adverse perinatal outcomes in low-risk pregnancies at or after 41 weeks gestation is very low with either induction or expectant management.

There is data to support the idea that induction and expectant management are not equivalent, rather induction conveys benefits to both the fetus and the mother. A meta-analysis by Grant reviewed 11 trials and concluded that a policy of routine induction had a lower rate of perinatal morbidity and cesarean section, demonstrating both fetal and maternal benefit when compared to expectant management. In addition, a recent review in the Cochrane Library concluded that routine induction in low-risk pregnancies at or after 41 weeks gestation is associated with a reduction in perinatal mortality, with no increase in the rate of instrument deliveries or cesarean section.

CERVICAL RIPENING AND INTRAPARTUM MANAGEMENT

Once the decision to deliver a patient has been made, the route of delivery and the specifics of intrapartum management will depend on individual circumstances, but a brief review of cervical ripening agents and potential complications of induction of labor is appropriate. A comprehensive review of all available methods for cervical ripening, indications, contraindications and dosing is beyond the scope of this chapter.

As many as 80 percent of patients who reach 42 weeks will have an unfavorable cervical exam (Bishop score less than 7) (Harris, 1983). There are many options for cervical ripening. The different preparations, indications and contraindications and multiple dosing regimes of each requires that the practitioner familiarize himself/herself with several of the preparations. Prostaglandin E2 gel and suppositories for vaginal application were used extensively until the late 1990s, when many pharmacies stopped manufacturing them due to the advent of commercially available and less labor intensive preparations. Chemical preparations, which are currently available, include, prostaglandin E1 tablets for oral or vaginal use, prostaglandin E2 gel for intracervical application, and a vaginal insert containing 10 mg of dinoprostone. One additional agent that may become available, but has been studied less than other pharmacologic agents, is the progesterone antagonist RU486.

Another method for ripening the cervix is by mechanical dilation. These devices may act by a combination of mechanical forces and by causing release of endogenous prostaglandins. Membrane sweeping or stripping (Foong, 2000), foley balloon catheters placed in the cervix (Sullivan, 1996), extra-amniotic saline infusions and laminaria have all been studied and been shown to be effective (Guinn, 2000).

Regardless of what method is chosen for cervical ripening, one ought to be aware of the potential hazards surrounding the use of these agents in the patient with a scarred uterus. In addition, the potential for uterine tachysystole and subsequent fetal distress requires that care be taken not to use too high of a dose or too short of a dosing interval in an attempt to get a patient delivered rapidly. Care should also be taken when using combinations of mechanical and pharmacologic methods of cervical ripening.

Once an induction of labor has begun, it is necessary to be aware of the major complications associated with inductions beyond 41 weeks, and have a plan for dealing with each of these potential complications. These include the presence of meconium, macrosomia and fetal intolerance to labor.

The farther pregnancy progresses beyond 40 weeks, the more likely it is that significant amounts of meconium will be present. This is due to the increased uteroplacental insufficiency, which leads to hypoxia in labor and activation of the vagal system. In addition, the presence of less amniotic fluid will increases the relative amount of meconium in utero. In order to counter the effects of meconium, a combination of amnioinfusion of normal saline and prompt suctioning of the oropharynx and nose upon delivery of the head must be considered.

Fetal macrosomia can lead to maternal and fetal birth trauma and arrest of labor, both the first and the second stages. Recognizing the limitations of ultrasound at term, it is still advisable to get an estimated fetal weight prior to induction of the postdates pregnancy. An acceptable alternative to ultrasound is estimation of fetal weight by abdominal palpation by an experienced practitioner, but an estimated fetal weight should be documented prior to beginning a postdates induction. In addition, mid-pelvic instrument deliveries should not be attempted. Perhaps the most important part of a delivery plan is being prepared for a shoulder dystocia, in the event that this unpredictable, anxiety provoking and potentially dangerous condition arises.

ANTEPARTUM FETAL SURVEILLANCE

Antepartum fetal surveillance should be used in postterm pregnancies (>42 wk gestation)when delivery is not performed (usually for obstetrical contraindications). Evidence to suggest that antepartum surveillance improves outcomes before 41 weeks gestation in low-risk pregnancies is insufficient, and routine use of antepartum surveillance between 40 and 41 weeks of gestation is not supported by the literature.

An amniotic fluid index of greater than 8 cm, and a reactive fetal heart rate tracing are reassuring. When performing the fetal heart rate tracing, the patient may be seated or in a lateral recumbant position with lateral tilt. Two accelerations in the fetal heart rate of 15 beats per minute over baseline in a 20 minute period constitute a reactive tracing. If the tracing is not reactive within the first 20 minutes, the test may be extended another 20 minutes. If the tracing remains nonreactive, a backup test must be performed in order to reassure oneself that the intrauterine environment is still safe. A contraction stress test or a full biophysical profile are both acceptable back-up tests. These may also be used if the tracing is reactive but has fetal heart rate decelerations. However, in the pregnancy that is beyond 41 weeks, the threshold for delivery should be very low.