Pregnancy due date calculator

From the first day of your last period we estimate your due date, how far along you are and the key dates of your pregnancy.

Updated September 10, 2026

First day of your last period

Cycle length

días

Days from one period to the next. 28 is typical.

The date comes from Naegele's rule: 40 weeks from your last period. It's a convention, not a prediction: only about 4% of babies arrive on their exact due date, and birth anywhere from week 37 to week 42 is considered normal. Dating by ultrasound from your OB-GYN or midwife takes precedence over this calculation.

Estimated due date

Enter the date to see the result.

How this calculator works

Enter the first day of your last period, or the date of conception if you know it, and adjust your cycle length if it isn’t 28 days. You’ll get your estimated due date, how many weeks and days pregnant you are, which trimester you’re in and the key dates of your pregnancy.

Naegele’s rule

The standard method dates from the 19th century and is still what doctors use for a first estimate: first day of your last period + 280 days. That’s 40 weeks, or the equivalent of adding nine months and seven days.

For example, if your last period started on January 1, 2026, your estimated due date is October 8, 2026.

Adjusting for your cycle

The rule assumes you ovulate on day 14 of a 28-day cycle. If your cycle is longer or shorter, ovulation shifts by the same number of days, and so does the due date. With a 35-day cycle, the same January 1 start gives a due date of October 15.

If you know the date of conception, the calculator subtracts two weeks to get the equivalent last period and counts from there: conceiving on January 15, 2026 also points to October 8.

How pregnancy weeks are counted

Weeks are counted from the first day of your last period, not from conception. It sounds odd, but it means that when you conceive you’re already considered about two weeks pregnant. “12 weeks” in your provider’s notes means 12 weeks since that last period.

First trimester: through the end of week 13.
Second trimester: weeks 14 to 27.
Third trimester: from week 28 until birth.

Babies born from week 37 are considered early term, from week 39 full term, and from week 42 post-term.

How accurate is it?

It’s a convention, not a prediction. Only about 4% of babies are born on their due date, and most arrive within two weeks either side of it. Dating by early ultrasound is more precise, so once you’ve had one, the date your OB-GYN or midwife gives you takes precedence over this calculation.

This calculator is for information only and doesn’t replace prenatal care. If you have any doubt about your dates or your pregnancy, talk to your provider.

Frequently asked questions

How is my due date calculated?

With Naegele's rule: add 280 days (40 weeks) to the first day of your last menstrual period. If your last period started on January 1, 2026, your estimated due date is October 8, 2026.

Why count from my last period and not from conception?

Because most people know when their last period started but not the day they conceived. Pregnancy weeks are counted from the last period by convention, which is why at conception you're already considered about two weeks pregnant. If you do know the conception date, for example after IVF, the calculator can start from it.

Does cycle length change the due date?

Yes. The standard rule assumes a 28-day cycle with ovulation on day 14. With a longer cycle you ovulate later, so the due date moves later too: a 35-day cycle pushes the January 1 example from October 8 to October 15.

How accurate is the due date?

It's an estimate. Only about 4% of babies arrive on their exact due date, and birth anywhere from 37 to 42 weeks is common. An early ultrasound, especially in the first trimester, is more accurate, and if it disagrees with this calculation your provider will go by the ultrasound.

When does each trimester start?

The first trimester runs through the end of week 13, the second from week 14 through week 27, and the third from week 28 until birth. The calculator shows which one you're in and the key dates along the way.