Due Date Calculator

What this calculator tells you

Enter one date and this tool estimates your due date, then fills in the picture around it: how far along you are in weeks and days, your current trimester, how many days remain, and the key milestone dates that mark the end of each trimester. You can calculate from whichever piece of information you have — your last period, a conception date, an IVF transfer, or an ultrasound measurement — and everything is worked out instantly, on your device.

It's the same arithmetic your provider uses to set a due date, made transparent so you can see exactly where the number comes from.

How due dates are estimated

The classic approach is Naegele's rule, and it's beautifully simple: a pregnancy is counted as 280 days — 40 weeks — from the first day of your last menstrual period (LMP). That's where the "you're 12 weeks pregnant" language comes from; gestational age is measured from the period, not from conception.

Why 280 days, and why from the period? Two reasons. First, the LMP is a date most people can pin down, whereas the exact moment of conception usually isn't known. Second, 280 days reflects the average length of a human pregnancy measured this way. Naegele's rule assumes a textbook 28-day cycle with ovulation around day 14 — so if your cycle is consistently longer or shorter, the estimate shifts. This calculator lets you set your average cycle length, and it adjusts the due date accordingly: a longer cycle means later ovulation, which pushes the due date back.

The four methods

Different situations call for different starting points, so the calculator offers four:

Last menstrual period (LMP)

Enter the first day of your last period and, optionally, your average cycle length. The due date is the LMP (adjusted for cycle length) plus 280 days. This is the default and works well when your cycles are regular and you remember the date.

Conception date

If you know roughly when conception happened, the calculator adds 266 days — fourteen fewer than from the LMP, because conception occurs about two weeks after the period that gestational age is counted from. Useful when you tracked ovulation or know the date intercourse led to pregnancy.

IVF transfer

For pregnancies from IVF, the embryo transfer date is a precise anchor because the age of the embryo is known. The calculator handles both common transfer types:

• A day-5 (blastocyst) transfer means fertilisation happened five days before the transfer.
• A day-3 (cleavage-stage) transfer means three days before.

It works back from the transfer to an equivalent last-period date, then applies the standard 280 days — so the estimate lines up with how clinics date IVF pregnancies.

Ultrasound

If a scan measured your gestational age on a particular date — say "8 weeks and 2 days on this date" — the calculator projects that forward to the due date. Early ultrasounds are often the most reliable dating method, so this is the one to use when a scan and your last period disagree.

Trimesters and milestones

Pregnancy is conventionally split into three trimesters, counted from the LMP:

• First trimester: weeks 1–13.
• Second trimester: weeks 14–27.
• Third trimester: weeks 28 to birth.

The calculator shows which trimester you're currently in and lists the dates each one ends, alongside the due date itself (the end of week 40). These milestones are handy for planning appointments, tests and the practical run-up to the birth — and for simply seeing the shape of the months ahead. To measure the exact gap between any two of those dates, the Date Difference Calculator breaks it down into days, weeks and months.

It also shows a progress figure: roughly what percentage of the 40-week journey you've completed today. Like everything here, that "today" is read from your own device the moment the page loads.

Why only about 4% are born on the exact date

It's worth saying clearly: a due date is an estimate, not an appointment. Only around one in twenty babies actually arrives on the predicted day. A pregnancy is considered full term anywhere from 37 to 42 weeks, and there's a wide, completely normal spread within that range — first babies, in particular, often arrive a little later than the date suggests.

So treat the due date as the midpoint of a window. It's genuinely useful for planning and for tracking how things are progressing, but a baby that comes a week or two on either side is following a perfectly ordinary timetable.

This is an estimate — talk to your provider

This calculator is for information and planning only; it is not medical advice and is no substitute for professional care. Your midwife or doctor will date your pregnancy using your history and, importantly, ultrasound measurements — and where their dating differs from a calendar estimate, theirs is the one to trust. If anything about your pregnancy concerns you, or if your own calculations and your provider's don't match, raise it with them.

Private by design — calculated in your browser

A pregnancy due date is about as personal as data gets, which is exactly why this tool keeps it on your device. Every figure — your due date, how far along you are, the trimester dates — is computed in your browser with plain JavaScript. Nothing you enter is sent to a server, logged or stored. There's no account, no tracking of your health details, and no third party in the loop.

That local-only design isn't a footnote; it's the point. You get the same answers a server-side calculator would give, without handing your last-period date or scan results to anyone. Enter your date, read your estimate, and close the tab knowing it stayed with you. And once the baby arrives, the Age Calculator will track their age down to the exact day.