OH-Blog Logo
Red baby shoes sitting on top of a calendar

How to Calculate Your Pregnancy Due Date

Baby’s coming, but when? Countdown to delivery starts with calculating the due date!

After learning that you’re pregnant, your next question is usually, “What’s my baby’s due date?” Most pregnancies last around 40 weeks. Your pregnancy due date is calculated by adding 280 days from the first day of your last period.

Remember, your due date is an estimate. Every pregnancy is different. It’s normal to deliver your baby within a couple of weeks before or after your due date. On average, only 5 percent of babies are born on their exact due dates.

Once you determine your due date, that doesn’t stop the questions or the learning. Read below for information on calculating your baby’s due date.

1. How can women with irregular periods calculate their due dates?

There are other ways your healthcare provider can calculate your due date if your menstrual cycle is irregular. An ultrasound early in your pregnancy can provide a more accurate due date. Your healthcare provider can also use the size of your uterus as a measurement. When you’re 20 weeks pregnant, the top of your uterus will be as high as your navel.

2. Can my due date change?

Yes, your due date can change, but it’s not something to get worried about. The most common reasons your due date can change are:

  • Irregular periods
  • Having your first ultrasound in the second trimester — ultrasounds in the first trimester provide the most accurate due date
  • An abnormal alpha fetoprotein level (AFP is a protein produced during pregnancy) — a high level can mean your due date was miscalculated

3. What other ways can OB/GYNs use to estimate due dates?

Another way your doctor may calculate your due date is to start with the first day of your last menstrual period, add seven days, then subtract three months. For example, if your last period started on May 1, adding seven days is May 8. Subtracting three months gives you a February 8 due date.

4. What happens if I’m past my due date?

Remember, your due date is an estimate. You’re not considered overdue until two weeks after your due date. Your doctor will likely check you twice a week once you’re a week overdue to make sure you and your baby are doing well. Your doctor may decide to induce labor after you’re one week overdue or wait to see if you go into labor on your own. If you haven’t gone into labor after two weeks, your doctor will probably induce labor.

Learn about the changes each trimester brings you and your baby.

Looking for an OB/GYN? We can help!

related articles