Although pregnancy is said to last nine months, healthcare providers track pregnancy by weeks and days. The estimated delivery date, also called the estimated due date, is calculated as 40 weeks or 280 days from the first day of the last menstrual period. Only 4 percent (1 in 20) women will deliver 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 last menstrual period. As many as 10 percent of pregnancies will deliver postterm.
Accurate dating — Accurate dating is essential to ensure that the pregnancy is in fact postterm. Ideally, an accurate gestational age is determined early in the pregnancy. In women who have regular menstrual periods, the date can often be reliably calculated based on the last menstrual period and a physical examination.
If there is uncertainty about the dates, or if the size of the woman's uterus is larger or smaller than expected based on the date of the last period, the gestational age of the fetus and due date can be estimated based upon findings on fetal ultrasound examination. This estimate is most accurate when performed early in pregnancy; ultrasounds performed in the last half of pregnancy are less reliable for estimating the due date.
POSTTERM PREGNANCY CAUSES
In most cases, the cause of postterm pregnancy is unknown. There are some factors that place a woman at increased risk. The incidence is higher in first pregnancies and in women who have had a previous postterm pregnancy. Genetic factors may also play a role. One study showed an increased risk of postterm pregnancy in women who were, themselves, born postterm.
However, variations in when a woman ovulates can lead to errors in calculating the true duration of pregnancy and lead to over- and underestimations of when the baby is due. An ultrasound examination performed in the first half of pregnancy is the most reliable method of calculating the date the baby is due, especially in women with long or irregular menstrual cycles.
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 risk is relatively small, with only 4 to 7 deaths per 1000 deliveries. By comparison, the risk of stillbirth or infant death in pregnancies between 37 and 42 weeks is 2 to 3 per 1000 deliveries.
Large body size — Postterm fetuses have a greater chance of developing complications related to larger body size (called macrosomia), which is defined as weighing more than 4500 grams, or about 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 — Also called "postmaturity syndrome," this refers to a fetus whose growth in the uterus after the due date has been restricted, usually due to a problem with delivery of blood to the fetus through the placenta.
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), and an increased rate of cesarean birth with its associated risks of bleeding, infection, and injury to surrounding organs.
POSTTERM PREGNANCY TREATMENT
Antenatal fetal monitoring — In most cases, a healthcare 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 or benefits of allowing the pregnancy to continue.
The American College of Obstetricians and Gynecologists has stated that it is only necessary to start antenatal fetal monitoring after 42 weeks (294 days) of gestation, although many obstetric care providers will start fetal testing at 41 weeks. Many experts recommend twice weekly testing, including a measurement of amniotic fluid volume. Testing may include observing the fetus' 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 the fetus' health. It consists of five components, nonstress testing and ultrasound measurement of four fetal parameters: fetal body movements, breathing movements, fetal 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.
Amniotic fluid volume is an important variable in the BPP because a low volume (called oligohydramnios) may be a sign of changes in the feto-uteroplacental circulation. Amniotic fluid level can become reduced within a short time period, even a few days.
Contraction stress test — A contraction stress test (CST) can also be done to assess fetal health. It involves giving an intravenous medication (oxytocin) to the mother to induce uterine contractions. The fetus' heart rate is monitored in response to the contractions. A fetus whose heart rate slows down during a CST may require an emergency cesarean delivery.
Inducing labor — The optimal time to deliver a baby in a woman who is postterm is sometimes hard to determine. The healthcare provider and woman 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 a woman's pregnancy. Inducing labor is more likely to fail in women whose cervix is not dilated or thinned, which could require the woman to undergo cesarean birth.
Most healthcare providers will induce labor if it does not begin spontaneously by 41 to 42 weeks of gestation. For a woman whose 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. Cervical change may also be accomplished using mechanical methods such as a Foley catheter bulb. Most women, 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 delivery is usually performed.
Some patients may choose to have a cesarean delivery, especially if the fetus is macrosomic (defined as an estimated fetal weight of greater than or equal to 4500 grams [about 10 pounds]), if they have a history of previous cesarean delivery, or for reasons of personal choice. It is important to understand the risks and benefits of cesarean delivery, and to discuss these issues with the physician who will be performing the procedure. (See "Patient information: C-section (cesarean delivery) (Beyond the Basics)".)
Some postterm infants have a distinctive appearance. The 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. Postterm newborns 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.
- 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 in women 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, and the fetus may develop problems due to having restricted space in the uterus or inhaling meconium (bowel movement). The increased size of the fetus can lead to complications, some of which can affect the mother during delivery. (See 'Postterm pregnancy risks' 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.)
- For most women, labor is induced if it has not occurred by 41 to 42 weeks. Some women will elect to have a cesarean delivery if the fetus has grown to a very large size. (See 'Inducing labor' above.)
- Some postterm infants have a distinctive appearance, with long and thin arms, dry and sometimes loose-appearing skin, and longer hair and nails. Postterm infants tend to be very alert at birth. (See 'Postterm infants' above.)
WHERE TO GET MORE INFORMATION
Your healthcare 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 healthcare 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 information: When your baby is overdue (The Basics)
Patient information: 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 information: 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 delivery of at risk pregnancies
Prenatal assessment of gestational age and estimated date of delivery
The following organizations also provide reliable health information.
- National Library of Medicine