The pregnancy due date is among the most important information for the pregnancy. Both a baby born too early or a baby born too much after the due date will potentially increase complications. In addition, decisions made during pregnancy rely on know the right due date.

Typically, a woman's due date is calculated 266 days after fertilization and ovulation or 280 days (or 40 weeks) after the first date of the last 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).

For some time now, the ACOG definition of a "term pregnancy" has changed. According to the updated defintion, a full term pregnancy is the 3-week period between 39 weeks and 0 days to 40 weeks and 6 days:

  • Preterm: Before 37 weeks and 0 days
  • Early term: Between 37 weeks and 0 days and 38 weeks and six days of gestation
  • Full term: Between 39 weeks and 0 days to 40 weeks and 6 days
  • Late term: the period between 40 weeks and 0 days to 40 weeks and 6 days
  • Postterm: the period at 42 weeks 0 days and beyond)

Officially, postdates or postterm 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.

Risks of postterm pregnancies

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.

Fetal and neonatal risks 

  • Reduced placental perfusion and not enough amniotic fluid
  • Meconium aspiration syndrome
  • Macrosomia
  • Shoulder dystocia
  • Increased forceps assisted or vacuum assisted birth

Maternal risks of postterm pregnancies

  • Increased labor induction
  • Increased forceps assisted or vacuum assisted birth
  • Increased Cesarean delivery with bleeding, infection, abnormal wound healing, abnormal placenta in future pregnancies, and rarely death

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.

Recent studies such as this one have found that pregnancies that are induced at 39 weeks "..are not associated with maternal or perinatal complications and may reduce the need for Cesarean section, risk of hypertensive disease of pregnancy and need for neonatal respiratory support."  Most obstetricians agree that a pregnancy should not proceed beyond 41 weeks and when a woman goes post-date 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.

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