Due Date Calculator – When Is My Baby Due?

How the due date is estimated

Most due dates are calculated with Naegele's rule: add 280 days (40 weeks) to the first day of your last menstrual period (LMP). This assumes a 28-day cycle with ovulation on day 14. If your cycles are longer or shorter, the calculator shifts the date by the difference.

Due date = LMP + 280 days + (cycle length − 28)

If your last period started on March 1 and your cycle is 28 days, your estimated due date is December 6. With a 32-day cycle it moves four days later, to December 10.

Pregnancy weeks and trimesters

Pregnancy is counted from the LMP, about two weeks before conception, so at conception you are already "two weeks pregnant". Trimesters are usually defined as:

First trimester: weeks 1–13
Second trimester: weeks 14–27
Third trimester: weeks 28–40 and beyond

How accurate is a due date?

Only about 4% of babies are born on their exact due date. Most arrive between 38 and 42 weeks. An early ultrasound, ideally before 14 weeks, measures the baby directly and is the most accurate way to date a pregnancy; your provider may adjust your due date based on it. If you conceived through IVF, the transfer date gives a more precise estimate.

This calculator is for information only. Always follow the dates and advice of your midwife or doctor.

Frequently asked questions

How is a due date calculated?

The standard method adds 280 days to the first day of your last menstrual period, adjusted for your cycle length.

Can my due date change?

Yes. If an early ultrasound measurement differs from your LMP date by more than about a week, your provider will usually update the due date.

What if I do not know my last period date?

An ultrasound can estimate gestational age from the baby's size. In the first trimester it is accurate to within about 5–7 days.

Last reviewed Oct 20, 2026. Results are estimates; check important decisions with a qualified professional.