The Pregnancy Calendar: Due Date Details

Think your due date is set in stone? Think again. Learn how your due date is calculated and why it’s important.

Congratulations. You’re pregnant! Are you like some women who know the precise moment their baby was conceived? Or are you like most, who are less certain of the exact date of conception?

During your first prenatal clinic visit, your doctor or midwife will help you calculate when this likely happened, an important factor in predicting your “estimated due date” (EDD) or estimated date of delivery.

How long does pregnancy last?

Pregnancy is measured in weeks. Doctors count forward 40 weeks from the first day of your last menstrual period to get your due date. Contrary to popular belief, a typical pregnancy lasts longer than nine months. Forty weeks is actually closer to 9 1/2 months. Pregnancy is considered “normal” if it lasts anywhere between 38 and 42 weeks.

You’re not actually pregnant until the day you conceive. But the date of ovulation is hard to pinpoint. So your EDD begins with the first day of your last period, which is estimated as two weeks prior to the date of conception. Your doctor may also conduct a pelvic exam and measure the size of your uterus (womb) and, if necessary, may also do an ultrasound (also called a sonogram). These exams are especially helpful in determining your EDD if you don’t know the date of the first day of your last period.

Will I deliver on my due date?

Because the EDD is just that — an estimate — only 1 in 20 babies arrive on their scheduled due dates. Most babies are born in the timeframe of two weeks before to two weeks after their EDD.

Also, most women do not have a “textbook” 28-day menstrual cycle. A normal menstrual cycle can last anywhere from 23 to 35 days. To calculate your due date, doctors use a 28-day cycle as a guideline. And even if you do have a regular cycle, your baby may be ready to be born before or after the 40-week mark.

Why is it important to know my due date?

Knowing your EDD helps you and your family plan for a new baby. Your due date tells doctors the gestational age of your unborn baby. A lot of health care decisions are based on this age. For example, prenatal tests are timed by the EDD. Your doctor also uses the EDD to determine how your pregnancy is going and to measure the growth and development of your baby.

For example, a pregnancy that reaches 42 weeks is considered post-term. If your baby is not born yet and you have passed your EDD, your doctor will talk with you about how he or she will check on you and the baby. Your doctor will advise you whether inducing labor is the next step. Knowing your EDD is one factor in this important decision.

Early prenatal care is important for every mom-to-be. Make an appointment with your doctor if you are planning to become pregnant, or as soon as you think you may be pregnant.

By Mary Small, Contributing Writer

Sources
American College of Obstetricians and Gynecologists. Your Pregnancy and Childbirth: Month to Month. 6th ed. Washington, DC: ACOG; 2015.
FamilyDoctor.org. Pregnancy: What to expect when you’re past your due date. Accessed: July 15, 2016.
American College of Obstetricians and Gynecologists. What to expect after your due date. Accessed: July 15, 2016.
KidsHealth. For parents. Pregnancy calendar. Accessed: July 15, 2016.

Last Updated: July 15, 2016