Due Date Calculator

Why the count starts before you were pregnant

Pregnancy is famously 40 weeks. It is also famously nine months, which is not the same thing, and the reason for the mismatch is the strangest convention in obstetrics: the clock starts on the first day of your last period, roughly two weeks before you conceived.

So on the day the egg was fertilised, you were already officially "two weeks pregnant". By the time a test turns positive, you are usually around four weeks. Nobody designed this to confuse you. They designed it around the only date most people can actually name. Almost no one knows the hour they ovulated, but most can say when their last period started, so medicine picked the knowable date and built the count on it. Every scan, screening and appointment you will have is scheduled against that clock.

This tool applies that convention. Below is how it turns your date into a due date, what the trimester dates mean, the assumption sitting under all of it, and why the answer deserves less reverence than it usually gets.

Two ways in, one piece of arithmetic

You can enter either the first day of your last period or your conception date, and the tool works out the other one before doing anything else. It hinges on a single 14-day step:

• Give it your last period, and it puts conception 14 days later.
• Give it your conception date, and it puts your last period 14 days earlier.

From there, everything is counted from conception: the due date is conception plus 266 days. That is the same as your last period plus 280 days, which is the classic version of the rule, named after Franz Naegele, a German obstetrician who died in 1851. The arithmetic has not changed since.

So for a last period starting 5 January 2026, the tool puts conception at 19 January and the due date at 12 October 2026. Count it either way and you land on the same day: 280 days from the period, or 266 from conception.

The trimester dates it gives you

Alongside the due date you get two staging posts. Using the same 5 January example, the end of the first trimester lands on 30 March (12 weeks along) and the end of the second on 13 July (27 weeks along). Both are counted in the standard way, from your last period, not from conception.

Fair warning that trimester boundaries are softer than they look. Different sources draw them in slightly different places, and the American College of Obstetricians and Gynecologists runs the first trimester a bit longer, through 13 weeks and 6 days. So treat these as signposts rather than gates. Nothing about a pregnancy changes because a calendar page turns.

The assumption underneath the number

Worth knowing, because it is not a small one, and the professional body that maintains the convention says it plainly. Dating a pregnancy at 280 days from the last period assumes a regular 28-day cycle with ovulation on day 14. It makes no allowance for cycle lengths that are not 28 days, for ovulation that wanders, or for the fact that only about half of women recall their last period date accurately.

The practical consequence: if your cycles run to 35 days, you ovulated around a week later than the rule assumes, and your real due date sits roughly a week later than the number here. This tool never asks for your cycle length, so it cannot make that adjustment. Our Conception Date Calculator does ask, which makes its conception estimate the better one if your cycles are not 28 days.

Which brings up the thing that outranks all of this arithmetic. An early ultrasound measures the embryo directly and dates the pregnancy to within about 5 to 7 days, better than any calculation from a remembered date. It is common for that scan to move the due date: in one trial, 40 percent of women given a first-trimester scan had their date changed because it disagreed with the period-based estimate by more than 5 days. If your clinic's date differs from this page, theirs is the one to write down.

A due date is a range wearing a disguise

It arrives as a single day on a calendar, which is exactly the wrong way to hold it. Only around 4 to 5 percent of babies are born on their due date. The overwhelming majority arrive somewhere in the weeks either side, and anything from 37 to 42 weeks is normal territory.

A more useful way to carry it is as a window, something like two weeks before to one week after. That framing spares a lot of unnecessary worry when the date comes and goes, which for first babies it very often does. Sort the non-negotiable preparations well before the date and leave the rest loose.

And the honest note: this is an estimate produced by arithmetic on one date. It is not a medical assessment, and it does not replace your midwife or doctor, who will confirm or revise the date properly. Babies, for their part, have never once consulted a calculator.

Questions people ask

My doctor gave me a different due date. Who is right?

Your doctor. If the date came from an early ultrasound, it beats any calculation from a remembered period, and clinical guidance says to go with the scan when the two disagree by more than a few days in the first trimester. This page runs the standard formula; a scan measures the actual pregnancy.

What if my cycles are irregular or I do not know my last period?

Then this method is on shaky ground, and it is worth saying so rather than dressing the answer up. The whole formula rests on a 28-day cycle and a date you can name. If either is missing, an early scan is not just better, it is really the only reliable option, and it is worth booking sooner rather than later.

Is the conception date it shows real?

In last-period mode, no, it is an assumption. The tool simply places conception 14 days after your period started, because that is what the convention says. It is not a measurement of anything, and if your cycle is not 28 days, it will be off by however much your ovulation differs from day 14.

References

Where the figures come from. The 280-day convention, its stated assumptions about cycle length and day-14 ovulation, the finding that around half of women recall their last period accurately, and the 40 percent redating figure after first-trimester ultrasound all come from the ACOG committee opinion on estimating the due date. The point that only about 4 percent of babies arrive on the estimated date, and that a range is the more useful way to quote it, draws on the Medscape review of gestational assessment.

1. American College of Obstetricians and Gynecologists. Methods for estimating the due date. Committee Opinion No. 700. Obstetrics & Gynecology. 2017;129:e150-e154. acog.org
2. Evaluation of Gestation. Medscape Reference. https://emedicine.medscape.com/article/259269-overview