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?

Why do women deliver past their due dates?
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 7 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.

What are the risks of a late pregnancy?
According to the ACOG, 95 percent of babies born between 42 and 44 weeks are born safely. But going past the due date does carry some risks, including:

• The placenta's ability to provide baby with adequate oxygen and nutrients may be compromised
• The volume of essential amniotic fluid may decline as baby grows (increases possibility of a pinched umbilical cord)
• The possibility of fetal distress increases
• The baby could grow too large to pass safely through the birth canal (also known as macrosomia)

With post-term pregnancies, there's also an increased possibility that the baby could have a bowel movement while still in the womb, putting him in danger of inhaling the waste product known as meconium.

Some of these complications can lead to a cesarean delivery which, as a surgical procedure, carries its own risks.