Assessment of the post-term pregnancy

Assessment of the post-term pregnancy – includes patient information sheet

Michael J. McMahon

A term pregnancy is completed in 38 to 42 weeks. “Post-term,” “prolonged,” “postdate” and “postmaturity” are terms that have been used to define a pregnancy that lasts beyond 42 weeks of confirmed gestational age. The incidence of post-term pregnancy is 3 to 12 percent, varying because of inaccurate dating or irregular ovulation.[1,2]

The perinatal mortality rate increases after 40 weeks, doubles by 42 weeks and, at 44 weeks, is four to six times greater than the mortality rate of the term gestation.[2-4] Maternal, fetal and neonatal morbidity also increase when a pregnancy extends beyond 42 weeks.[2-5] Maternal complications include an increased risk of cesarean birth with associated sequelae, such as intraoperative complications, hemorrhage from uterine atony, endometritis and wound infection. The risk of cesarean birth more than doubles after 42 weeks’ gestation.[3] Other maternal complications include trauma and hemorrhage resulting from vaginal delivery of macrosomic infants.[6] A fetal mortality rate of about 2 per 1,000 exists in pregnancies extending beyond 290 days, despite prenatal testing.[7] Fetal and neonatal complications from the post-term pregnancy can include macrosomia, shoulder dystocia, brachial plexus injuries and meconium aspiration.[3,4]

Women with a post-term pregnancy have a 30 percent chance of delivering post-term in a subsequent pregnancy, while women with two previous post-term pregnancies have a 40 percent chance of a subsequent post-term delivery.[8]

The use of obstetric ultrasound assessment over the past decade has led to many advances in perinatal medicine, including the management of the post-term pregnancy. In women unsure of the dates of their last menstrual period or in pregnancies with a size/date discrepancy, first-trimester ultrasonography can help the clinician estimate the gestational age. In a documented post-term pregnancy, ultrasound evaluation of amniotic fluid in addition to nonstress testing enables assessment of fetal well-being.

Assessment of Gestational Age

An accurate assessment of gestational age is important because clinical decision-making relating to dating can be affected throughout the pregnancy. Nowhere is this more important than in the management of a pregnancy presumed to be post-term. Without an accurate account of the length of gestation, unwarranted antenatal testing and intervention are possible.


Conceptional age is usually known with certainty only in patients who have undergone in vitro fertilization or artificial insemination. Gestational age, on the other hand, is defined as beginning with the last normal menstrual period. Clinical dating parameters include an evaluation of the first day of the last normal menstrual period, the first-trimester pelvic examination, the fundal height measurements, the date of quickening and the fetal heart tone documented by Doppler ultrasound or fetoscope.

The most reliable clinical predictor of gestational age is an accurately dated last menstrual period.[9] However, the use of menstrual history to date gestation can be misleading. It has been suggested that approximately 20 to 40 percent of women cannot recall the first day of their last menstrual period and are uncertain of the date of conception.[10] Even in women who do remember the first day of their last menstrual period, the date may be unreliable as a predictor of true conceptional age because of oligomenorrhea, vaginal bleeding early in pregnancy as a result of implantation, ovulation occurring under 11 or beyond 21 days into the menstrual cycle, or recent use of oral contraceptive pills.

In one study,[11] it was shown that the first day of the last normal menstrual period predicted the due date ([+ or -] 14 days) in 85 percent of cases, if an optimal menstrual history could be obtained. An optimal history was defined as certainty of the first day of the last menstrual period, documentation of regular menstrual cycles, no irregular vaginal bleeding and no use of oral contraceptive pills for two months before conception. Unfortunately, an optimal history could only be obtained in 55 percent of the study population.[11]

An early pelvic examination (before 12 weeks of gestation) by an experienced practitioner is both reproducible and accurate.[12] Quickening, or the first perception of fetal movement by the mother, occurs at around 19 weeks in first pregnancies and at approximately 17 weeks in subsequent pregnancies.[12] Audible fetal heart tones are another way to clinically evaluate gestational age. Electronic Doppler can detect fetal heart sounds by 11 to 12 weeks. A fetoscope is able to detect fetal heart sounds by 19 weeks. Although the last normal menstrual period appears to be the most precise predictor of gestational age, early pelvic examination, fundal height measurement, and detection of quickening and fetal heart tones by Doppler or fetoscope can help confirm gestational age (Table 1).[9] In the uncomplicated, low-risk patient who presents in the first trimester of pregnancy, gestational age can be obtained clinically, and ultrasound evaluation is not likely to be of any significant clinical benefit.


Clinical Estimates of Gestational Age

Weekly nonstress tests for the assessment of fetal well-being in the post-term pregnancy have a false-negative rate of 6.1 per 1,000. A false-negative nonstress test is defined as a stillbirth within a week of a reactive test result. When the nonstress test is performed twice weekly, the false-negative rate has been demonstrated to decrease to 1.9 per 1,000.[20] It has also been shown that spontaneous variable decelerations on a nonstress test may indicate cord compression and should not be ignored. An increased rate of poor perinatal outcome has been reported following this occurrence.[21]


The biophysical profile is primarily used to assess the status of the fetus when the screening nonstress test is nonreactive or equivocal. The biophysical profile consists of a nonstress test and ultrasound evaluation of fetal breathing, fetal movement and fetal tone, and quantification of amniotic fluid volume.[22]

Not all settings have an experienced clinician capable of performing the evaluation. In the evaluation, two points are assigned to each of the following variables: a reactive nonstress test, one or more episodes of rhythmic fetal breathing movement of 30 seconds or more in 30 minutes, three or more discrete body or limb movements in 30 minutes, one or more episodes of extension of a fetal extremity with return to flexion, and the detection of a single pocket of amniotic fluid exceeding 2 cm in two perpendicular planes. For each part of the biophysical profile, a score of zero is assigned if any of the following is not detected: a reactive nonstress test, fetal breathing movements, fetal movements, fetal tone or adequate amniotic fluid volume.

A score of 8 to 10 is normal. A score of 4 to 6 is considered equivocal and requires fetal reevaluation within 12 to 24 hours or performance of a contraction stress test. A biophysical profile score of zero to 2 strongly correlates with fetal hypoxemia and warrants immediate delivery. The false-negative rate for the biophysical profile is 7 per 1,000, comparable to the rate for the weekly nonstress test.[22] A twice-weekly biophysical profile and intervention for low amniotic fluid volume decreases this risk to zero per 1,000.[23]

The most important part of the biophysical profile in the management of a postterm pregnancy is the amniotic fluid volume, taken in conjunction with the results of the nonstress test. Amniotic fluid is largely composed of fetal urine. As placental function decreases, perfusion of fetal organs, such as the kidneys, decreases and can lead to a reduction of amniotic fluid. In a post-term pregnancy, umbilical cord compression has been shown to be associated with fetal compromise. If oligohydramnios or decreased amniotic fluid are present, the potential exists for antenatal or intrapartum fetal compromise.

Amniotic fluid volume is measured by the amniotic fluid index. The amniotic fluid index is determined by measuring the maximal vertical fluid pocket depth in centimeters (without umbilical cord loops) in each of the four uterine quadrants, using real-time ultrasonography. The volumes are added together to obtain the amniotic fluid index (Figure 2). After 41 weeks of gestation, the amniotic fluid index is considered low and suggestive of poor placental function if it is less than 5 cm; borderline if it is between 5 cm and 8 cm, and normal if it is greater than 8 cm.[24]

Management of the Post-term Pregnancy

Induction of labor should be considered in the management of a patient with a known gestational age of 42 weeks or beyond. The Bishop scoring system for elective induction is shown in Table 2. Recent studies have shown a decrease in the rate of cesarean birth when induction is performed, compared with the rate with serial antenatal testing, but no difference in the incidence of perinatal mortality or neonatal morbidity.[2,25] In patients with a favorable cervix, induction of labor is recommended, using either artificial rupture of membranes or oxytocin (Pitocin). Sweeping (stripping) of the membranes has also been shown to be an effective method of inducing labor in post-term women with a favorable cervix.[26]


The Authors

MICHAEL J. MCMAHON, M.D., M.P.H. is assistant professor of obstetrics and gynecology at the University of North Carolina at Chapel Hill. Dr. McMahon completed an internship and a residency in obstetrics and gynecology at Indiana University School of Medicine, Indianapolis. He earned a master’s degree in public health at the University of North Carolina, where he was a National Research Service Award Fellow at the Cecil G. Sheps Center for Health Services Research.

JEFFREY A. KULLER, M.D. is assistant professor of obstetrics and gynecology at the University of North Carolina at Chapel Hill. Dr. Kuller is also the director of Reproductive Genetics, medical director of the Maternal Serum AFP Program and assistant residency director of obstetrics and gynecology at the University of North Carolina at Chapel Hill. He completed an internship and a residency in obstetrics and gynecology at the Johns Hopkins Hospital, Baltimore. He also completed a fellowship in maternal-fetal medicine at the University of Pittsburgh School of Medicine Magee-Women’s Hospital, and a fellowship in medical genetics at the University of California, San Francisco, School of Medicine.

JEROME YANKOWITZ, M.D. is assistant professor of obstetrics and gynecology at the University of Iowa, Iowa City, where he is director of the Fetal Diagnosis and Treatment Center. He completed an internship and a residency in obstetrics and gynecology at the Johns Hopkins Hospital. He also completed a combined fellowship in maternal-fetal medicine and medical genetics at the University of California, San Francisco, School of Medicine.

Address correspondence to Michael J. McMahon, M.D., M.P.H., University of North Carolina School of Medicine Department of Obstetrics and Gynecology, Division of Maternal-Fetal Medicine, CB #7570, 214 MacNider Bldg., Chapel Hill, NC 27599-7570.

COPYRIGHT 1996 American Academy of Family Physicians

COPYRIGHT 2004 Gale Group