Due Date Calculator

Estimate your due date from your last period or conception date.

How the due date is estimated

• From your last period (Naegele's rule): due date = first day of your last period + 280 days (40 weeks). If your cycle is longer or shorter than 28 days, the date moves by the difference.
• From conception: due date = conception date + 266 days (38 weeks).

Pregnancy weeks are counted from the first day of the last period, which is about two weeks before conception.

Worked example

• First day of last period January 10, 2027 with a 28-day cycle: due date October 17, 2027, conception around January 24, the second trimester starts April 18 and the third July 25.
• With a 35-day cycle, the same period gives October 24, 2027.
• Conception on January 24, 2027 also gives October 17, 2027.

Frequently asked questions

How accurate is a due date?

It is an estimate. Only a small minority of babies are born on their exact due date, and most arrive in the weeks before or after it.

What if my cycles are irregular?

The estimate from your last period may be less reliable. Your doctor or midwife can confirm the date with an ultrasound, and they may use that date instead.

Why is pregnancy counted as 40 weeks if conception happens about two weeks after my period starts?

Doctors count from the first day of the last period because it is usually known, while the exact conception date is not. So a pregnancy lasts about 38 weeks from conception and 40 weeks from the last period.

Does this replace a visit to my doctor?

No. Use it for planning and curiosity. A healthcare provider confirms your dating and cares for you during pregnancy.

This calculator is for general information only and is not medical advice.