Calculating Your Baby’s Due Date

Calculating Baby Due Date

Congratulations on your pregnancy! What an exciting time! From the moment you learn you are expecting, it’s fun to think about each phase of the baby’s development and to estimate when your baby is due. 

How to Calculate Your Baby’s Due Date

People most often refer to pregnancy as a nine-month process, but the average pregnancy lasts 280 days, or 40 weeks. Calculating a due date is far from an exact science, however, as babies often arrive earlier or later than this time frame. 

A pregnancy is considered full-term anywhere between 37-42 weeks from the first day of your last menstrual period. A simple method to estimate your due date is to add seven days to the date of your last period, and then add nine months to this date. 

If you are unsure about the date of your last period, you can take your best guess, but the only way to have more certainty around your baby’s due date is to get an ultrasound to determine how far the embryo has developed. You are eligible for an ultrasound anytime after the first six weeks of your pregnancy.

You can also calculate your due date based on the date of conception, which you may or may not know. To use this method, count out 38 weeks from the approximate (or exact) date the baby was conceived. 

Even when you know your due date, or even after your baby is born, there are fun calculator tools you can use to figure out the date your baby was conceived. Sites like can help you backtrack from your due date, first day of your last menstrual period, and/or the baby’s birth date. 

How Accurate is a Due Date Calculator?

There are several “due date calculators” online that promise to tell you the exact date your baby is due, but even this fun tool can only provide an estimate. Only 1 in 20 women deliver on their calculated due date, so any method you use is only an educated guess.

It’s also possible the egg wasn’t fertilized on the same day you were sexually active, as eggs can be fertilized up to two days after sexual activity. This is why doctors (and pregnancy calculators) typically use the date of your last menstrual period, rather than an estimated conception date, to determine a baby’s due date. 

Your baby’s due date may also be adjusted as your pregnancy progresses when an ultrasound reveals information like the size of the fetus and the fetus’ development. 

You can find online pregnancy calculators on sites like and The American Pregnancy Association

Week-to-Week Pregnancy Calculator

For a routine pregnancy, at least one ultrasound is usually performed, which is always a much-anticipated event for expectant parents! It’s the first peek at the miracle of your new baby. In some instances, like higher-risk pregnancies or for people with certain medical conditions, ultrasounds may be performed more frequently.

You may also wonder about how the baby is developing week by week! It’s fun to think about whether there are little toes, eyes, or arms growing or – of course – when the baby’s gender can be determined, should you choose to find out in advance. 

A week-by-week pregnancy calculator can help you find out what new developments are happening, how big the fetus is, and how many days/week pregnant you are at any given time.

These weekly calculators are available online at sites like and

There are many changes that happen to Mom’s body during pregnancy, and learning about how the baby is developing and growing along the way is fun to know, too! We hope you found this explanation of calculating your baby’s due date helpful but please do not hesitate to reach out to our office if you have further questions. 


Are you looking for an OB/Gyn you can trust? Do you live in the Tucson area? Book an appointment today!