How to Calculate Your Due Date

Pregnancy is a long journey whose result can affect your life considerably. Couples who are in the process of getting pregnant are most interested in knowing as much as they can about their pregnancy. After you confirm whether you’re pregnant, the most common question you will be asked during your pregnancy is “When’s your due date?”

Of course, you could go to an OB GYN clinic and consult with a professional, but essentially, this is really just a simple math problem.

The average duration of pregnancy is 40 weeks, or 280 days, starting from the first day of your last normal menstrual period. If you know that date, you’re all set. If not, try to remember the last day of your last normal menstrual period, then subtract the length of your usual period in days. For most women, this is about five days, give or take two.

A quick way to estimate this is to note that 40 weeks is about nine months and seven days. So if you add this length of time to the first day of your last normal menstrual period, you’ve got a pretty good idea of when your child will be born.

For example, let’s set the first day of your last normal menstrual period was March 18. If you add seven days, you get March 25. Count nine months from March, and you’re in December. So the expected due date of this pregnancy would be on December 25. Looks like you’re getting more than just presents and greeting cards this Christmas.

However, this calculation is merely an estimate. Few women give birth exactly on their due date. Nevertheless, about 80 percent of births happen within 10 days of the due date. Any pregnancy occurring between 37 and 42 weeks is considered a full term pregnancy.

Preterm deliveries occur because of preterm labor, and can be associated with several factors, such as multiple pregnancy (twins or more), placenta previa, thyroid disease, tobacco or other drug use, infection, or comorbid diseases such as hypertension and diabetes. Women who have had previous preterm deliveries have an increased risk in their current pregnancy ending sooner than expected.

Postterm pregnancies are usually harmless for the mother. However, the fetus may have certain complications. The placenta is not designed to last forever, so beyond the 42nd week, its functionality in providing oxygen and nutrients to the fetus declines. Postterm fetuses are at risk for several problems, most notably aspiration pneumonia, wherein the fetus passes fecal material called meconium into the amniotic fluid, and the fluid travels into its lungs at the time of delivery.

Your due date provides vital information that you and your OB GYN can use to plan ahead and track the progress of your pregnancy. You can correlate the changes your body goes through with your gestational age, and know whether something is amiss, or just a part of the process.