How to Calculate Your Pregnancy Due Date

Your due date helps your doctor track your unborn baby’s growth and development. They’ll use your due date to make sure you and your baby are getting checkups and tests at the ideal stages of your pregnancy.

The pregnancy countdown usually starts on the first day of your last period. That was technically before you were even pregnant!

A full-term pregnancy is 280 days, or about 40 weeks. (This is about 9.2 months, but it includes that time before you were pregnant.)

Calculating your estimated due date involves some math—and it can get a little tricky. There are a few different methods you can use:

Due Date Calculator Method 1: If You Know the First Day of Your Last Period

Used since the 1800s, this is the most common way of estimating a due date: Start with the date your last period started. Add 280 days. To do this, you can count ahead exactly 40 weeks on a calendar.

Or start with the date your last period started. Subtract 3 calendar months. Add 1 year and 1 week. This should give you the same date as above. It’s just a different way to get there.

For example, if your last period was June 1, subtract three months, then add one year, then add one week, and you should get March 8 of the following year.

Due Date Calculator Method 2: If You Know the Conception Date

Conception typically happens within 12–24 hours of ovulation. So, if you were tracking it, use the date of ovulation to give you a slightly more precise estimated due date.

Start with the date of conception or ovulation. Add 266 days.

Due Date Calculator Method 3: If You Had IVF

Estimating due dates around in vitro fertilization (IVF) tends to be more accurate than some other methods, since the calculation is based on the exact known date of transfer.

Start with the IVF transfer date. Add 266 days. Subtract the age of the embryos.

Example: For a 3-day embryo transfer, subtract 3 days.

Due Date Calculator Method 4: Ultrasound Dating

Your doctor will likely perform an ultrasound in the beginning of your pregnancy, by the end of week 13.

This ultrasound scan is used to measure the length of the fetus from crown to rump (head to bum!). This provides the most accurate estimated due date. This will be compared to the due date based on your last period.

What Can Affect Your Due Date

An estimated due date can vary based on a variety of factors, such as:

• When you ovulated. The 280-day pregnancy is based on a typical 28-day menstrual cycle. But many people have shorter or longer cycles. Some online due date calculators also allow you to input your average cycle length to better estimate an ovulation or conception date.
• Irregular periods. If your cycle changes often, it can be more challenging to know what date you ovulated and determine an exact due date.
• How the baby is measuring. Your doctor may change your due date if the fetus is bigger or smaller than expected.
• Expecting multiples. With twins, triplets, and other multiples, pregnancy is still counted starting with the first day of the last menstrual period. But most twins are born by the end of week 37.

Remember: The due date is just an estimate. Many factors can affect exactly what day your baby arrives.

Only 5% of babies are born on their due date.

No matter what day you meet your baby, it’s sure to be a memorable one.

Sources

The American College of Obstetricians and Gynecologists. (2017) Methods for Estimating the Due Date.

Better Health Channel. Department of Health, State Government of Victoria, Australia. Overdue babies. Accessed August 14, 2023.

CARE Fertility. (2017) Calculating your IVF due date.

Cheong-See, F. et al. (2016) Prospective risk of stillbirth and neonatal complications in twin pregnancies: Systematic review and meta-analysis. BMJ.

Cleveland Clinic. Due Date Calculator. Accessed August 14, 2023.

Johns Hopkins Medicine. Calculating a Due Date. Accessed August 14, 2023.

Johnson Memorial Health. (2015) 3 Ways to Calculate Your Baby’s Due Date.

March of Dimes. Calculating your due date. Accessed August 14, 2023.

Texas Health. Due Date Calculator. Accessed August 14, 2023.