Due Date Calculator — estimate your pregnancy due date

How it works

Doctors count a pregnancy from the first day of your last menstrual period (LMP), not from conception — because the LMP is a date you can usually identify, while the exact moment of conception is hard to pin down. By this convention, according to the Office on Women's Health, "pregnancy lasts about 40 weeks", which is 280 days from the LMP. That is the estimated due date (also called Naegele's rule).

Conception normally happens about two weeks after the LMP, at ovulation, so the first two weeks of the count occur before you are actually pregnant. That is why the estimated conception date is roughly LMP + 14 days.

The 40 weeks are grouped into three trimesters: weeks 1–12, weeks 13–28 and weeks 29–40. The calculator marks when the second and third trimesters begin so you can see the shape of the pregnancy at a glance.

A due date is an estimate: few babies arrive on the exact day, and cycle length and early ultrasound measurements can shift it. Treat it as the center of a likely window, and confirm dating with your healthcare provider.

Worked example

For a last period beginning January 1, 2026, add 280 days to reach a due date of October 8, 2026. Conception is estimated around January 15, 2026 (two weeks in), the second trimester begins March 26, 2026 (week 13) and the third begins July 16, 2026 (week 29).

Frequently asked questions

How is a due date calculated?

It is counted from the first day of your last menstrual period (LMP): pregnancy lasts about 40 weeks, so the due date is LMP + 280 days. This is known as Naegele's rule. For an LMP of January 1, 2026, the due date is October 8, 2026.

Why is pregnancy dated from the last period, not conception?

The first day of the last period is a date most people can identify, whereas the exact time of conception usually is not. Dating from the LMP gives a consistent starting point, which is why clinical due dates and gestational age are measured this way.

How accurate is a due date?

It is an estimate of the middle of a likely delivery window, not a fixed appointment. Few births happen on the estimated date itself; most occur in the weeks around it. An early ultrasound can refine the date.

When does each trimester start?

Counting from the LMP, the first trimester is weeks 1–12, the second is weeks 13–28 and the third is weeks 29–40. The calculator shows the calendar dates when your second and third trimesters begin.

Does my cycle length change the due date?

It can. The 280-day rule assumes ovulation around day 14 of an average 28-day cycle. If your cycles are much longer or shorter, ovulation — and therefore your due date — may shift by a few days, which a provider can adjust for.

What if I know my conception date instead?

If you know when you conceived, the due date is about 266 days later, since conception is roughly two weeks after the LMP. A conception-based estimate and an LMP-based one should agree to within a couple of weeks.