Learn about the risks and the monitoring methods available.

Why Are Some Women Overdue?

Full-term pregnancies range from 38 to 42 weeks. But what if your pregnancy continues beyond 42 weeks? It's not unheard of -- in fact, according to the American College of Obstetricians and Gynecologists, 10 percent of all pregnancies go past 42 weeks. But why is the process longer for some?

Some pregnancies are post-term. But as it turns out, others are actually the result of the incorrect assignment of a due date. Due dates are tricky because it's hard to pinpoint the exact age of a fetus. Reasons for this include irregular periods, sketchy or inaccurate menstrual history presented to the obstetrician, and mistaking spotting during very early pregnancy for a period. Doctors usually use several methods together to make their best estimate of a due date, including:

  • Calculation based on your last ovulation (the most reliable method).
  • Calculation based on the first day of your last menstrual period.
  • Clinical examination of the uterus for size.
  • Your first detection of fetal movement (the fetus usually makes its first movements between 16 and 20 weeks).
  • Fetal heartbeat (in normal pregnancies, the doctor can detect it between 18 and 20 weeks).
  • Ultrasound, which during early pregnancy can estimate fetal age within seven to 10 days (it's not as effective later in the pregnancy).

Unfortunately, if you have irregular cycles it could prove difficult to accurately predict a due date.

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