Typically, a woman's due date is 266 days after fertilization or ovulation or 280 days or 40 weeks) after the first date of the menstrual period.
Most women will deliver between 37-42 weeks of the pregnancy (259 to 294 days after the LMP if she has a 28-day cycle or 245 to 280 after ovulation).
Officially, postdates is defined as a pregnancy that goes to 42 weeks or more than 14 days or 2 weeks after the due date. Once you go more than a week or two past your due date, the risk of pregnancy complications increases, both for your baby and for you. To lower the risk of post-date pregnancies, many doctors think it's a good idea to induce labor, especially when the cervix is already open as your is.
Because of the risk associated with going past your due date, estimating the correct date is one of the most important elements of prenatal care.
Your doctor will estimate your due date (emphasis on the word "estimate") based on the dates of your last menstrual period, the usual length of your periods, and a physical exam.
Any method of estimating due dates has a certain margin of error, but they're usually a pretty good guide. So when a woman goes past her assigned due date, her doctor will first review her prenatal chart to ascertain whether the date was correctly calculated. It's not unusual to find such an error; only about 5 percent of all pregnancies end exactly on the assigned date and up to 10 percent go more than two weeks past the date.
Although the cause for post-date pregnancy usually isn't determined, your risk increases if it happened in a previous pregnancy, if you're older, or if you have a baby with anencephaly.
A post-date pregnancy can lead to these additional problems:
- The fetus may grow too big (macrosomia), increasing the likelihood of a cesarean section or fetal injury.
- The placenta may provide less nutrition to the fetus (placental insufficiency), thus decreasing the amount of amniotic fluid (oligohydramnios) and slowing the fetus's growth or causing it to lose weight.
- Under stress, the fetus may pass its intestinal waste, called meconium, into the amniotic fluid and swallow it. If meconium is aspirated into the lungs, it can block air passages and cause breathing problems or pneumonia.
Obstetricians don't completely agree on exactly how to proceed when a woman goes post-date, but the choice is usually between inducing labor or testing the fetus and placenta to see if problems are developing. To help with the decision, your doctor will start checking your cervix around 41 weeks to see if it's opening up and starting to efface. Inducing labor when the cervix is not yet dilated or shortened increases the risk for C-section. If the cervix is closed, some doctors induce labor only when fetal tests are abnormal.
If there are no other risk factors, most doctors will start checking fetal and placental health starting at 41-42 weeks. The tests include the following:
If any of the tests is abnormal, your doctor will usually deliver the baby. If your cervix is already open at 41 weeks, most doctors will induce labor rather than take a chance and wait for an abnormal fetal test, because induction is more likely to lead to a vaginal delivery.