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INTRODUCTION — Although pregnancy is said to last nine months, health care providers track pregnancy by weeks and days. The estimated delivery date, also called the estimated due date or EDD, is calculated as 40 weeks or 280 days from the first day of the last menstrual period (LMP). Only 4 percent (1 in 20) of pregnant people will give birth on their due date.
The normal duration of pregnancy is 37 to 42 weeks, which is referred to as "term." A postterm pregnancy, also called a prolonged pregnancy, is one that has extended beyond 42 weeks or 294 days from the first day of the LMP. As many as 10 percent of pregnant people give birth postterm.
IMPORTANCE OF ACCURATE PREGNANCY DATING — Accurate dating is essential to ensure that the pregnancy is really postterm. Ideally, an accurate gestational age is determined early in the pregnancy. Among individuals who have regular menstrual periods, the date can often be reliably calculated based on the last menstrual period (LMP) and a physical (pelvic) examination.
In individuals with long or irregular menstrual cycles, variations in when the individual ovulates can lead to errors in calculating the true duration of pregnancy based on LMP and lead to over- and underestimations of when the baby is due.
If menstrual periods are not regular or there is uncertainty about the LMP or the size of the uterus is larger or smaller than expected based on the date of the last period, then the gestational age of the fetus and due date are best estimated based upon findings on fetal ultrasound examination. This estimate is most accurate when performed early in pregnancy (up to approximately 20 weeks of gestation); ultrasounds performed in the last half of pregnancy are less reliable for estimating the due date.
POSTTERM PREGNANCY CAUSES — Inaccurate dating based on the last menstrual period is the most common cause of postterm pregnancy. In accurately dated pregnancies, the cause of postterm pregnancy is usually unknown. There are some factors that place an individual at increased risk. The incidence is higher in first pregnancies and in individuals who have had a previous postterm pregnancy. Genetic factors seem to also play a role. One study showed an increased risk of postterm pregnancy in pregnant individuals who were, themselves, born postterm.
POSTTERM PREGNANCY RISKS — Pregnancy that continues beyond 42 weeks is associated with risks to the fetus and the mother.
Risks to the fetus
Stillbirth or neonatal death — The incidence of stillbirth or infant death is increased in pregnancies that continue beyond 42 weeks. However, the actual risk is relatively small, with 4 to 7 deaths per 1000 births. By comparison, the risk of stillbirth or infant death in pregnancies between 37 and 42 weeks is 2 to 3 per 1000 births.
Large body size — Postterm fetuses usually continue to grow after the due date, so they have a greater chance of developing complications related to larger body size and macrosomia (macrosomia is defined as a baby weighing more than 4500 grams, or approximately 10 pounds). Complications can include prolonged labor, difficulty passing through the birth canal, and birth trauma (eg, fractured bones or nerve injury) related to difficulty in delivering the shoulders (shoulder dystocia).
Fetal dysmaturity — Some postterm fetuses stop gaining weight after the due date. "Dysmaturity" or "postmaturity" syndrome refers to a fetus whose weight gain in the uterus after the due date has stopped, usually due to a problem with delivery of blood to the fetus through the placenta, leading to malnourishment.
After birth, these infants have a distinctive appearance. Their arms and legs may be long and thin. The skin may appear dry and parchment-like, with peeling and sometimes meconium staining. The skin may appear loose, especially over the thighs and buttocks. Scalp hair may be longer or thicker, and the fingernails and toenails may be long. They are typically very alert, and may have a "wide-eyed" look.
Few studies have examined long-term outcomes (eg, growth and development patterns, intelligence) of postterm infants. In general, the outcome appears similar in both postterm and term infants.
Meconium aspiration — Beyond term, the fetus is more likely to have a bowel movement, called meconium, into the amniotic fluid. If the fetus is stressed, there is a chance it will inhale some of this meconium stained amniotic fluid; this can cause breathing problems when the baby is born.
Risks to the mother — Risks to the mother are related to the larger size of postterm fetuses and include difficulties during labor, an increase in injury to the perineum (including the vagina, labia, and rectum) at vaginal birth, and an increased rate of cesarean birth with its associated risks of bleeding, infection, and injury to surrounding organs.
POSTTERM PREGNANCY PREVENTION AND TREATMENT
Antenatal fetal monitoring — In most cases, a health care provider will recommend tests on the fetus if the pregnancy extends beyond the due date. These tests give information about the health of the fetus and about the risks of allowing the pregnancy to continue.
These tests are begun at or beyond 41 weeks of gestation. Many experts recommend twice weekly testing, including a measurement of amniotic fluid volume. Testing may include observing the fetus's heart rate using a fetal monitor (called a nonstress test) or observing the baby's activity with ultrasound (called a biophysical profile).
Nonstress testing — Nonstress testing is done by monitoring the baby's heart rate with a small device that is placed on the mother's abdomen. The device uses sound waves (ultrasound) to measure changes in the baby's heart rate over time, usually over a period of 20 to 30 minutes. Normally, the baby's baseline heart rate should be between 110 and 160 beats per minute and should increase above its baseline by at least 15 beats per minute for 15 seconds several times during the test.
The test is considered reassuring (called "reactive") if two or more fetal heart rate increases (called "accelerations") are seen within a 20 minute period. Further testing may be needed if these increases are not observed after monitoring for 40 minutes.
Biophysical profile — A biophysical profile (BPP) score is calculated to assess fetal health. It consists of five components, nonstress testing and ultrasound measurement of four fetal parameters: body movements, breathing movements, tone (flexion and extension of an arm, leg, or the spine), and amniotic fluid volume. Each component is scored individually, 2 points if normal and 0 points if not normal. The maximum possible score is 10. Sometimes the nonstress testing component of the test is omitted. In these cases, the maximum possible score is 8.
Amniotic fluid volume is an important variable in the BPP because a low volume (called oligohydramnios) may be a sign of changes in the fetoplacental circulation. Amniotic fluid level can become reduced within a short time period, even a few days.
Inducing labor — The optimal time to deliver a baby in a pregnancy that is postterm is sometimes hard to determine. The health care provider and pregnant individual must consider the risks and benefits of continuing the pregnancy, the results of antenatal testing, and the condition of the cervix (the lower part of the uterus, which opens into the vagina). Normally, the cervix begins to dilate (open) and efface (thin) towards the end of pregnancy. Inducing labor is more likely to take a long time when the cervix is not dilated or thinned.
Most health care providers will induce labor if it does not begin spontaneously by 41 to 42 weeks of gestation. When the cervix is not favorable, labor can be induced with a medication applied directly to the cervix or in the vagina, which causes the cervix to soften and dilate. Sometimes medication is given by mouth. Cervical change may also be accomplished using mechanical methods such as a Foley catheter bulb.
Most pregnant people, including those whose cervix is favorable, will also require an intravenous medication (oxytocin), which stimulates the uterus to contract; uterine contractions further stimulate cervical dilation and effacement. If induction of labor does not completely dilate and efface the cervix, or if complications develop that require the baby to be delivered quickly, a cesarean birth is usually performed.
Some individuals may choose to have a cesarean birth, especially if the fetus is very large (eg, >5000 grams or 11 pounds), or they have a history of previous cesarean birth, or for reasons of personal choice. It is important to understand the risks and benefits of cesarean birth and to discuss these issues with the physician who will be performing the procedure. (See "Patient education: C-section (cesarean delivery) (Beyond the Basics)".)
SUMMARY
●A postterm pregnancy is one that extends beyond 42 weeks (294 days) from the first day of the last menstrual period; as many as 10 percent of pregnancies are postterm.
●The chance of postterm pregnancy is higher in first pregnancies and especially in pregnant individuals who have had a postterm pregnancy in the past. Genetic factors may play a role. (See 'Postterm pregnancy causes' above.)
●There are certain risks associated with postterm pregnancy. The chance of stillbirth or infant death increases slightly. Most postterm fetuses continue to grow, and the large size can cause problems during birth. Infrequently, the fetus may not continue to grow and may become malnourished. Beyond term, the fetus is more likely to have a bowel movement, called meconium, into the amniotic fluid. (See 'Risks to the fetus' above.)
●Risks to the mother include difficulties during labor, an increase in injury to the perineum at vaginal birth, and an increased rate of cesarean birth. (See 'Risks to the mother' above.)
●Tests are used to monitor the health of a postterm fetus and to determine whether it is safe to allow the pregnancy to continue. (See 'Antenatal fetal monitoring' above.)
●In most pregnancies, labor is induced if it has not occurred by 41 to 42 weeks. Some individuals will elect to have a cesarean birth if the fetus has grown to a very large size. (See 'Inducing labor' above.)
WHERE TO GET MORE INFORMATION — Your health care provider is the best source of information for questions and concerns related to your medical problem.
This article will be updated as needed on our web site (www.uptodate.com/patients). Related topics for patients, as well as selected articles written for health care professionals, are also available. Some of the most relevant are listed below.
Patient level information — UpToDate offers two types of patient education materials.
The Basics — The Basics patient education pieces answer the four or five key questions a patient might have about a given condition. These articles are best for patients who want a general overview and who prefer short, easy-to-read materials.
Patient education: When your baby is overdue (The Basics)
Patient education: Shoulder dystocia (The Basics)
Beyond the Basics — Beyond the Basics patient education pieces are longer, more sophisticated, and more detailed. These articles are best for patients who want in-depth information and are comfortable with some medical jargon.
Patient education: C-section (cesarean delivery) (Beyond the Basics)
Professional level information — Professional level articles are designed to keep doctors and other health professionals up-to-date on the latest medical findings. These articles are thorough, long, and complex, and they contain multiple references to the research on which they are based. Professional level articles are best for people who are comfortable with a lot of medical terminology and who want to read the same materials their doctors are reading.
Shoulder dystocia: Risk factors and planning birth of high-risk pregnancies
Oligohydramnios: Etiology, diagnosis, and management in singleton gestations
Postterm pregnancy
Prenatal assessment of gestational age, date of delivery, and fetal weight
The following organizations also provide reliable health information.
●National Library of Medicine
(https://medlineplus.gov/healthtopics.html)
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