Due Date by Conception Calculator — from conception to due date

How it works

Pregnancy due dates are normally counted as 40 weeks (280 days) from the first day of the last menstrual period (LMP). But conception does not happen until ovulation, which the Office on Women's Health places about two weeks after the LMP. So from the moment of conception, the remaining time is 280 − 14 = 266 days, or 38 weeks.

The calculator adds 266 days to your conception date to estimate the due date. It also works backward 14 days to show the implied last period, which is the date a standard LMP-based calculator would use — the two methods agree to within a couple of weeks.

The 40 weeks are grouped into trimesters (weeks 1–12, 13–28 and 29–40 from the LMP), and the calculator marks when your second and third trimesters begin. As always, a due date is an estimate; an early ultrasound refines it.

Worked example

For a conception date of January 15, 2026, add 266 days to reach a due date of Oct 8, 2026. Working back 14 days gives an implied last period of January 1, 2026 — the same due date an LMP-based estimate would produce, since 266 + 14 = 280 days.

Frequently asked questions

How do you calculate a due date from conception?

Add 266 days (38 weeks) to the conception date. Pregnancy is 40 weeks from the last period and conception is about two weeks after it, so 38 weeks remain from conception. Conceiving on January 15, 2026 gives a due date of October 8, 2026.

Why 266 days and not 280?

The 280-day (40-week) figure is counted from the last menstrual period, which is about two weeks before conception. Subtracting those two weeks leaves 266 days from conception to the due date.

Is a conception-based due date more accurate?

If you know the conception date precisely — for example from ovulation tracking or IVF — it can be very accurate. Otherwise, an early ultrasound is generally the most reliable way to date a pregnancy.

How does this compare to a last-period due date?

They should agree closely. This calculator shows the implied last period (conception − 14 days), which is exactly what an LMP-based calculator uses, so both give the same due date when the dates line up.

When do the trimesters start?

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

How accurate is any due date?

A due date marks the middle of a likely delivery window, not a fixed day. Only about 1 in 20 babies arrive exactly on it; most come in the weeks around it.