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.
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.
Depending on your condition, and that of your baby, your doctor may discuss the possibility of inducing labor. This is a decision that must be made on a case-by-case basis. Remember that most babies are not born on their actual due date. While complications could potentially occur in a postdate pregnancy, they're rare. Most of the time, babies are born healthy and complications are minimal. This is the result of the excellent fetal monitoring techniques available to your health-care provider.
While you might feel uncomfortable and anxious to meet your new baby, rest assured that a postdate pregnancy is not uncommon. Keep the lines of communication open with your health-care provider. It might help to write down your questions so you can discuss them at your visits. She can inform and reassure as she monitors your progress.
http://health.discovery.com/centers/pregnancy/americanbaby/pastduedate_02.html