How to Calculate Your Due Date

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Learn more about calculating your due date and, once you have calculated it, what that date actually represents for your baby’s arrival.

Calculate Due Date

From the minute you first noticed early pregnancy signs or saw your positive home pregnancy test, you have probably been thinking about when you will meet your baby. Human gestation is typically 40 weeks or 280 days, but normal pregnancies can come in well below and above that average length. Read on to learn more about calculating your due date and, once you have calculated it, what that date actually represents for your baby’s arrival.

How to calculate your due date

The first thing that is tricky about pregnancy duration is that medical convention dates pregnancies from the date of your last menstrual period, regardless of when you actually conceived, which is around the time of ovulation. You can manually calculate your due date in one of two ways. If you have a calendar handy—digital calendars work, though this is easier with a paper calendar—find the start date of your last menstrual period, that is, the day you began bleeding. From that date, count forward on the calendar 40 weeks. You can also subtract three months from the first day of your last menstrual period, then add seven days, and jump forward a year to arrive at your estimated due date. For example, if your last menstrual period was June 28, going back three months in time would give you March 28, and adding seven days to that date would give you April 4. Jump forward a year, and you know that your estimated due date is April 4 of the next calendar year. The date you just calculated is your estimated due date, and likely the date you will hear from your doctor or midwife when it is time for your first prenatal appointment.

Your care provider may use a “pregnancy wheel,” a calendar printed in a circle that can point to a predicted date based on the date of your last menstrual period. And you can automatically calculate your due date using any number of due date calculators online and in mobile apps. You can read more about some of your options in this Pregistry blog post, and if you are a member of Pregistry, you can use the due date calculator on the website.

But what does that due date really mean?

So once you have made your calculations or your favorite online tool has done so for you, what does it mean? Will your baby dutifully arrive on that exact day, like a rocket launching after a countdown? Unfortunately, probably not. Only about four in a hundred pregnancies result in birth on the actual due date, which is why it is probably better to think of the due date as an estimate. Care providers usually do not think much of it if a baby is born as early as 37 weeks (so three weeks earlier than the date you just calculated) or as late as 42 weeks (add fourteen days to your calculations). In the example above, your estimated due date range would be from March 14 to April 18—more than a month of possible baby arrival time!

One reason why the due date is just an estimate is that the length of human gestation could vary naturally, based on the genetics of the mother and any number of environmental factors. Another reason is that it is hard to know the exact date of conception. Although conception coincides with ovulation, sperm can live within the human body for up to five days, so as long as they meet an egg during that window, conception can occur. And while it is usually assumed that a woman ovulates on the fourteenth day of her cycle, the day of ovulation is naturally different in different women. Someone who ovulated on day 10 might have a baby earlier than someone who ovulated on day 14, even if their length of gestation is the same. And if you are someone with irregular periods, the first day of your last menstrual period may not be helpful in determining when your baby will be born.

Only about four in a hundred pregnancies result in birth on the actual due date, which is why it is probably better to think of the due date as an estimate.

It can certainly be a bit disappointing to realize that baby will likely not arrive exactly when you expect him or her to. But the due date range can also be a great opportunity to do everything you can to prepare—pack your bag, install your car seat, and narrow down your list of names—while realizing that not being in control of the situation is something that happens often in parenting. As uncomfortable as it might be, it’s certainly good practice.

Abby Olena
Dr. Abby Olena has a PhD in Biological Sciences from Vanderbilt University. She lives with her husband and children in North Carolina, where she writes about science and parenting, produces a conversational podcast, and teaches prenatal yoga.

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