You are in: eMedicine Specialties > Obstetrics and Gynecology > General Obstetrics Postterm PregnancyArticle Last Updated: Jun 19, 2006AUTHOR AND EDITOR INFORMATIONSection 1 of 7
  Author: Jennifer R Butler, MD, FACOG, Assistant Director, Department of Obstetrics and Gynecology, Division of General Obstetrics and Gynecology, Carolinas Medical Center Jennifer R Butler is a member of the following medical societies: Alpha Omega Alpha, American College of Obstetricians and Gynecologists, American Medical Association, and Association of Professors of Gynecology and Obstetrics Coauthor(s): Paul T Wilkes, MD, Assistant Professor, Department of Obstetrics and Gynecology, University of Nevada School of Medicine; Henry Galan, MD, Fellowship Director of Maternal and Fetal Medicine, Assistant Professor, Department of Obstetrics and Gynecology, University of Colorado Health Science Center Editors: Bryan D Cowan, MD, Professor and Chairman, Department of Obstetrics and Gynecology, University of Mississippi College of Medicine; Consulting Staff, Department of Obstetrics and Gynecology, Veterans Affairs Medical Center; Medical Director, Wiser Hospital for Women, University of Mississippi Medical Center; Francisco Talavera, PharmD, PhD, Senior Pharmacy Editor, eMedicine; Richard S Legro, MD, Professor, Department of Obstetrics and Gynecology, Division of Reproductive Endocrinology, Pennsylvania State University College of Medicine; Consulting Staff, Milton S Hershey Medical Center; Frederick B Gaupp, MD, Consulting Staff, Department of Family Practice, Assumption Community Hospital; Lee P Shulman, MD, Professor of Obstetrics and Gynecology, Feinberg School of Medicine, Northwestern University; Chief, Division of Reproductive Genetics, Department of Obstetrics and Gynecology, Prentice Women's Hospital, Northwestern Memorial Hospital Author and Editor Disclosure Synonyms and related keywords: postterm pregnancy, postmaturity syndrome, due date, calculated gestational age, crown-rump length, CRL, composite biometry, biparietal diameter, BPD, head circumference, abdominal circumference, femur length, transcerebellar diameter, fetal monitoring, post date pregnancy INTRODUCTIONSection 2 of 7
  Postterm pregnancies define pregnancies extending up to or after 42 weeks. The reported frequency is approximately 3-12%. The most frequent cause of postterm pregnancy is inaccurate dating criteria. Additional risk factors include primiparity, prior postterm pregnancy, male gender of the fetus, and genetic factors. Laursen et al studied monozygotic and dizygotic twins and their subsequent development of prolonged pregnancies. They found that maternal but not paternal genetic factors influenced the rate of postterm pregnancies and accounted for the etiology in as many as 30% of these pregnancies. Although the last menstrual period (LMP) has been traditionally used to calculate the estimated due date (EDD), many inaccuracies exist using this method in women who have irregular cycles, have been on recent hormonal birth control, or who have first trimester bleeding. Ultrasonographic dating early in pregnancy can improve the reliability of the EDD; however, it is necessary to understand the margin of error reported at various times during each trimester. A calculated gestational age by composite biometry from a sonogram must be considered an estimate and must take into account the range of possibilities. For example, crown-rump length (CRL) is 3-5 days, ultrasonography performed at 12-20 weeks of gestation is 1 week, at 20-30 weeks is 2 weeks, and after 30 weeks is 3 weeks. Thus, a pregnancy that is 35 weeks by a 31-week ultrasound could actually be anywhere from 32 weeks to 38 weeks (35 wk +/- 3 wk). If the calculated ultrasonographic gestational age varies from the LMP more than the respective range of error, it is used instead to establish the final EDD. The importance of determining by what method a pregnancy is dated cannot be overemphasized because this may have significant consequences if the physician delivers a so-called term pregnancy that is not or observes a so-called term pregnancy that is very postterm. When determining a management plan for an impending postterm pregnancy (>40 wk of gestation but <42 wk), the first decision is whether to deliver a patient and, if so, when and by what route. If the physician decides not to deliver, the decision whether to institute antepartum fetal surveillance and what method(s) of surveillance to use must be discussed with the patient. Note that if the pregnancy is at risk for an adverse outcome from an underlying condition, either maternal or fetal, inducing labor may proceed without documented lung maturity. Also, an elective induction of labor may proceed at or after 39 weeks of gestation in the absence of documented lung maturity provided that 36 weeks have elapsed since documentation of a positive human chorionic gonadotropin (+hCG) test finding, 20 weeks of fetal heart tones have been established by a fetoscope or 30 weeks by a Doppler examination, or 39 weeks' gestation have been established by a CRL or by an ultrasound performed before 20 weeks of gestation consistent with dates by the patient's LMP. TIMING OF DELIVERYSection 3 of 7
The first decision that must be made when managing an impending postterm pregnancy is whether to deliver. In certain cases (eg, nonreassuring surveillance, oligohydramnios, growth restriction, certain maternal diseases), the decision is straightforward. 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, there are frequently several options to consider when determining a course of action in the low-risk pregnancy. The certainty of gestational age, cervical examination findings, estimated fetal weight, patient preference, and past obstetric history must all be considered when mapping a course of action. Perinatal morbidity and mortality do not increase appreciably between 40 and 41 weeks of gestation; however, the perinatal mortality rate at gestational ages greater than 42 weeks is double that at term and increases 6-fold at 43 weeks. In addition, pregnancies extending up to or beyond 42 weeks (postterm pregnancies) are at risk for macrosomia, shoulder dystocia, cephalopelvic disproportion, and dysmaturity syndrome. Dysmaturity syndrome affects 20% of postterm fetuses and is thought to be caused by chronic uteroplacental insufficiency resulting in oligohydramnios, meconium aspiration, and reversible neonatal complications. Maternal risks include an increase in labor dystocia, perineal injuries, and cesarean deliveries. These complications support the idea that well dated pregnancies should not be allowed to progress beyond 42 weeks of gestation, but the question of how a pregnancy between 41 and 42 weeks should be managed remains. Delivery versus expectant management of low-risk pregnancies at 41 weeks of gestation has recently been addressed in the literature. The main argument against a policy of routine induction of labor at 41-42 weeks has been that induction increases the rate of cesarean delivery 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 were not necessarily postterm. In addition, the potential for increasing the risk for cesarean delivery 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 delivery rates in women entering spontaneous labor and those who had induced labor. Herabutya et al, the National Institute of Child Health and Human Development, and the Canadian Multicenter Postterm Pregnancy Trial have completed 3 prospective randomized studies. No increase in the rate of cesarean delivery was found in patients who were randomized to routine induction of labor. In fact, more cesarean deliveries 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. Further data suggest that induction may actually be more beneficial than expectant management in patients at 41 weeks of gestation. 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 delivery, demonstrating both fetal and maternal benefit 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 delivery. In summary, routine induction at 41 weeks' gestation does not increase the cesarean delivery rate, and may decrease it, without negatively affecting perinatal morbidity or mortality. In fact, there may be both maternal and neonatal benefits to a policy of routine induction of labor in well-dated low-risk pregnancies at 41 weeks' gestation. A policy of routine induction at 40 weeks' has few benefits, and there are multiple reasons not to allow a pregnancy to progress beyond 42 weeks. CERVICAL RIPENING AND INTRAPARTUM MANAGEMENTSection 4 of 7
Once the decision to deliver a patient has been made, the route of delivery and the specifics of intrapartum management depend on individual obstetric circumstances, and 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 article. As many as 80% of patients who reach 42 weeks' gestation have an unfavorable cervical examination (ie, Bishop Score <7). Many options are available for cervical ripening. The different preparations, indications, contraindications, and multiple dosing regimes of each require practitioners to familiarize themselves 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 because of the advent of commercially available and less labor-intensive preparations. Currently available chemical preparations include prostaglandin E1 tablets for oral or vaginal use (misoprostol), prostaglandin E2 gel for intracervical application (Prepidil), and a prostaglandin E2 vaginal insert (Cervidil). Cervidil contains 10 mg of dinoprostone and has a lower constant release of medication than Prepidil. In addition, this vaginal insert device allows for easier removal in the event of uterine hyperstimulation. Many studies have compared the efficacy and risks of various prostaglandin cervical ripening agents. Rozenburg et al performed a randomized trial comparing intravaginal misoprostol and dinoprostone vaginal insert in pregnancies at high risk of fetal distress. They found that both methods were equally safe for the induction of labor and misoprostol was actually more effective. 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, Foley balloon catheters placed in the cervix, extra-amniotic saline infusions, and laminaria have all been studied and have been shown to be effective. Regardless of what method is chosen for cervical ripening, the practitioner must 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 to avoid using too high a dose or too short 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 watch for the major potential complications associated with inductions beyond 41 weeks' gestation and to have a plan for dealing with each. Complications 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 increased uteroplacental insufficiency, which leads to hypoxia in labor and activation of the vagal system. In addition, the presence of less amniotic fluid increases the relative amount of meconium in utero. Traditionally, saline amnioinfusion and aggressive nasopharyngeal and oropharyngeal suctioning at the perineum were used to decrease the risk of meconium aspiration syndrome. Recent studies contradict this standard practice. Fraser et al performed a prospective randomized multicenter study evaluating the risks and benefits of amnioinfusion for the prevention of meconium aspiration syndrome. They concluded that in clinical settings, which have peripartum surveillance, amnioinfusion of thick, meconium-stained amniotic fluid did not decrease the risk of moderate-to-severe meconium aspiration syndrome, perinatal death, or other serious neonatal disorders compared with expectant management. In addition, other recent studies have shown that deep suctioning of the airway at the perineum does not effectively prevent meconium aspiration syndrome, contrary to popular belief. Fetal macrosomia can lead to maternal and fetal birth trauma and to arrest of both first- and second-stage labor. Recognizing the limitations of ultrasound at term, it is still advisable to obtain an estimated fetal weight prior to induction of the postdate 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 postdate induction. In addition, mid-pelvic instrument deliveries should not be attempted. Perhaps the most important part of a delivery plan is being prepared for shoulder dystocia in the event that this unpredictable, anxiety-provoking, and potentially dangerous condition arises. Finally, intrapartum fetal surveillance in an attempt to document fetal intolerance to labor before it leads to acidosis is critical. Whether continuous fetal monitoring or intermittent auscultation is used, interpretation of the results by a well-trained clinician is of paramount importance. If the fetal heart rate tracing is equivocal, fetal scalp stimulation and/or fetal scalp blood sampling may provide the reassurance necessary to justify continuing the induction of labor. If the practitioner cannot find reassurance that the fetus is tolerating labor, cesarean delivery is recommended. ANTEPARTUM FETAL SURVEILLANCESection 5 of 7
Antepartum fetal surveillance is suggested in postterm pregnancies when delivery is not performed. Although no randomized prospective trials exist demonstrating a benefit of fetal monitoring, no proof exists that it negatively affects postterm pregnancies either. Despite a lack of evidence, antepartum fetal surveillance of postterm pregnancies has become an accepted standard of care despite a lack of consensus as to a specific regimen of surveillance to be offered. The perinatal mortality rate increases gradually throughout pregnancy, with the greatest risk affecting pregnancies continuing past 41 weeks. Therefore, although no evidence can prove that routine monitoring between 40 and 42 weeks improves perinatal outcome, ACOG states that it is reasonable to begin antepartum testing after 41 weeks gestation. No single method of antenatal surveillance has been shown to be superior to any other. Options include a nonstress test, contraction stress test, full biophysical profile, modified biophysical profile (nonstress test and amniotic fluid index), or a combination of these modalities. Evaluation of the amniotic fluid level has been shown to be especially important because of demonstrated increased adverse pregnancy outcomes. Therefore, delivery should be implemented in the event of oligohydramnios with or without other nonreassuring tests. Doppler ultrasonography has been shown to provide no proven advantage for evaluating postdate or postterm pregnancies and should not be routinely used. A modified biophysical profile has been shown to be as sensitive as a full biophysical profile. Boehm et al demonstrated that twice-weekly testing of patients at risk for fetal distress was superior to weekly testing, decreasing the rate of stillbirth from 6.1 per 1000 live births to 1.9 per 1000. In summary, the use of a nonstress test and an amniotic fluid index 2 times per week for pregnancies continuing past 41 weeks is reasonable. In addition, if any indication during antepartum surveillance leads the practitioner to question the intrauterine environment, delivery should be expedited. SUMMARYSection 6 of 7
The management of postterm pregnancies is complicated and fraught with complex issues. The decision of whether to induce labor or to proceed with expectant management with or without antepartum fetal surveillance is not taken lightly. Data support inducing labor at 41 weeks' gestation in an accurately dated, low-risk pregnancy, regardless of cervical examination findings. This strategy, although not without its critics, averts the need for antepartum fetal surveillance and does not increase the cesarean delivery rate; in fact, it may decrease the cesarean delivery rate. REFERENCESSection 7 of 7
Article Last Updated: Jun 19, 2006 |
