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POSTDATE PREGNANCY
Waiting for the birth of a child is an exciting and anxious time. Most women (80%) give birth between 38 and 42 weeks of pregnancy. These pregnancies are called full-term.
However, only 5% of babies arrive on the exact "due date." An average of 10% of normal pregnancies extend beyond 42 weeks. These are called postterm or postdate pregnancies. There are two types of postdate pregnancies: those that are actually past their due dates and those that have been assigned incorrect due dates.
The reasons for postdate pregnancy are not clearly understood. Postdate pregnancies seem to occur as often in women who have given birth before as in women with first pregnancies.
Postdate Pregnancy: A Difficult Diagnosis
The average length of pregnancy (gestation) is 280 days, or 40 weeks, from the first day of the last menstrual period. When a pregnancy exceeds 294 days, or 42 weeks, it is considered postdate.
Knowing the gestational age of the fetus is the most important factor in diagnosing postdate pregnancy. It is very difficult, though, to pinpoint the age of the fetus. Inaccurate menstrual histories, irregular menstrual periods, and other causes may make it difficult to predict the exact due date. For this reason, health care providers often use more than one method to cross-check the gestational age of the fetus. The methods used most often include:
- Ovulation - If it is known, the time of ovulation is the most reliable method to determine the age of the fetus.
- Menstrual period - The due date is calculated from the first day of bleeding of the last menstrual period.
- Clinical exam - Throughout pregnancy, but especially early in pregnancy, the size of the uterus can provide useful information for determining gestational age.
- "Quickening" - Pregnant women usually first feel the fetus move, or "quicken," by 16-20 weeks.
- Fetal heart tones - In a normal pregnancy, fetal heart tones usually can be heard by the health care provider, using a special stethoscope, by 18-20 weeks.
- Ultrasound - This test, in which sound waves are used to create pictures of the fetus, is another useful method. Ultrasound can be used to estimate the age of a fetus within 7-10 days if it is done in the first half of pregnancy. Later in pregnancy, this method is not as accurate.
What are the Risks in a Postdate Pregnancy?
If gestation extends beyond 42 weeks, there are increased risks to the health of the fetus and mother. The incidence of loss increases after 42 weeks. It is important to remember that risks to the fetus, although real, occur in only a small number of postdate pregnancies.
Tests for Fetal Well-Being
If a postdate pregnancy is suspected, there are a number of tests that can help the health care provider monitor the well-being of the fetus and the environment inside the uterus. Generally, tests are begun at around 40-41 weeks of pregnancy. Some of these tests can be done by you, some are done in the health care provider's office, and others are done in the hospital.
A kick count is simply a record of how often you feel your baby move. Healthy babies tend to move about the same amount each day. A sudden decrease in movement should be reported to your health care provider right away, because it could mean fetal distress. Your health care provider may want to do more tests to decide if the baby needs to be delivered soon.
In electronic fetal monitoring, electronic instruments are placed on the mother's abdomen to record the fetus's heart rate in response to its own movements or to contractions of the mother's uterus. Two types of tests can provide reassuring information of the fetus's health and can give early warning if the fetus is in trouble:
- Nonstress test: This test measures the way a fetus responds to its own body movements. Normally, the fetal heart rate increases when the fetus moves. Each fetal movement felt by the mother or noted by the health care provider is marked on a paper recording of the fetal heart rate.
- Biophysical Profile: An ultrasound exam performed to assess the baby's well-being is called a biophysical profile. Ultrasound uses sound waves to create a two-dimensional picture of the fetus on a screen. This picture can show the position and size of the fetus and placenta, as well as the fetal heartbeat, breathing, and body movements. Ultrasound can also be used to measure the amount of amniotic fluid and to reveal important information about the placenta.
What If the Pregnancy Is Postdate?
If the fetus appears to be active and healthy and the amniotic fluid volume appears normal, mother and fetus may continue to be monitored at regular intervals until labor begins on its own. Your health care provider may decide to apply an ointment called prostaglandin gel to your cervix . This will help to thin or "ripen" your cervix and encourage labor. This is often used after 41 weeks if the cervix is thick and undilated ("unripe"). If the fetus appears to be in danger, it should be delivered, either by inducing labor or by cesarean birth. Labor can be induced (brought on) by giving a drug called oxytocin to cause uterine contractions.
Usually your health care provider will induce labor at 42 weeks, even if the baby does not appear to be in distress.
Finally . . .
It's important to remember that there could be increased risks to the fetus in a postdate pregnancy. That's why it is important for each patient and her partner to discuss these risks with her health care provider and to understand possible alternatives before delivery.
Most postdate babies are born healthy and delivered safely. Tests and careful monitoring during the last weeks of pregnancy and during labor can help ensure the best possible outcome.
Thomas G. Stovall, M.D.
Dr. Stovall is a Clinical Professor of Obstetrics and Gynecology at the University of Tennessee Health Science Center in Memphis, Tennessee and Partner of Women's Health Specialists, Inc.
Date Published: 2004-03-23
7800 Wolf Trail Cove, Germantown, TN 38138
Phone: (901) 682-9222; Fax: (901) 682-9505