Pregnancy Due Date Calculator

A due date is an estimate, and a fairly blunt one: it is the day you reach 40 weeks counted from the first day of your last period. Roughly 4% of babies are born on it, and most arrive somewhere in the two weeks either side. Its real job is not prediction but bookkeeping — it fixes the week count that every scan, screening and decision through the pregnancy is scheduled against.

How the due date is calculated

The standard method is Naegele's rule: add 280 days — 40 weeks — to the first day of your last menstrual period. An LMP of January 1, 2026 gives a due date of October 8, 2026. That is all the arithmetic there is.

The odd part is that it counts from your last period, which is roughly two weeks before you conceived. This is deliberate. The first day of bleeding is a date you can actually observe, while the moment of conception usually isn't, so obstetrics dates every pregnancy from the observable event and accepts the two-week head start. It is why you are considered "4 weeks pregnant" the day you miss a period, having conceived only two weeks earlier, and why this tool reports an estimated conception date two weeks after the LMP you entered — January 15 in the example above.

The consequence worth knowing: gestational age runs about two weeks ahead of the actual age of the embryo. When a scan says 12 weeks, the fetus has been developing for about 10. Every week number in this tool, in your notes, and in every pregnancy app is gestational age, so they are all consistent with each other — but none of them is the fetus's age.

Why cycle length changes the answer

Naegele's rule assumes a 28-day cycle with ovulation on day 14. If your cycles are longer or shorter you ovulate later or earlier, and the due date moves with it. The part that varies is the stretch before ovulation; the luteal phase from ovulation to the next period is close to a fixed 14 days for almost everyone. So the adjustment is simply the difference from 28, added to the due date:

• 21-day cycle, LMP January 1, 2026 → conception around January 8, due October 1, 2026 (a week earlier).
• 28-day cycle → conception around January 15, due October 8, 2026.
• 35-day cycle → conception around January 22, due October 15, 2026 (a week later).

A week is not a rounding error at the end of a pregnancy — it can be the difference between being offered an induction and not — so the cycle field is worth filling in accurately if your cycles are reliably not 28 days. If they are irregular, the number is a guess and an early dating scan will beat it.

The three ways in

The three modes are the same calculation entered from different ends, so any one of the dates fixes the other two.

• Last period is the default and the clinical standard. Use the first day of bleeding, not the last.
• Conception date is for when you actually know it — tracked ovulation, a single known date, or an IVF transfer. The due date is then 266 days later, and the cycle length is irrelevant, so that field disappears.
• Due date works backwards from a date a scan already gave you, filling in the week count and the whole milestone calendar. This is the mode to use after a dating scan, because the scan's date supersedes anything calculated from remembered dates.

Trimesters and milestones

Trimester boundaries are conventions rather than biological events, and sources differ by a week or so. This tool uses the common obstetric split: the first trimester runs to 13w6d, the second from 14w0d to 27w6d, and the third from 28w0d to birth. If another app puts you a few days the other side of a boundary, that is the convention differing, not an error.

The milestone list is dated the same way — in weeks and days from the same anchor — and covers the checkpoints that actually get scheduled: the missed period at 4 weeks, a visible heartbeat around 6, the end of the embryonic period at 8, cell-free DNA screening from 10, the nuchal translucency window at 11–13w6d, the 20-week anatomy scan, the 24-week viability threshold, Group B strep testing at 36–37 weeks, and the term windows at the end.

Those last ones are worth reading carefully, because "term" is not one thing. Early term is 37w0d–38w6d, full term is 39w0d–40w6d, late term is 41w0d–41w6d, and post term is 42 weeks and beyond. Birth at 37 weeks is no longer premature but it is not the same as full term; the 39-week window has the best outcomes, which is why elective deliveries are generally not scheduled before it.

How accurate is any of this

The calculation is exact; the inputs are not. An LMP-based due date rests on remembering a date correctly and on having ovulated when the rule assumes, and the second of those is frequently wrong — ovulation varies by several days even in regular cycles. A first-trimester ultrasound measuring crown-rump length is accurate to about ±5 days and is what providers use to set or correct the due date. If a scan and your dates disagree by more than about a week, the scan generally wins.

So treat the output here as a good working estimate for planning — when to book leave, when the anatomy scan will fall, roughly which month you are looking at — and let the dating scan be the authority. Once you have that date, come back and use the Due date mode to get a milestone calendar built on it.

What this doesn't do

This is a dating calculator, not a medical tool. It doesn't adjust for IVF transfer day (a day-5 blastocyst transfer dates differently from a day-3 one — enter the conception date as the transfer date minus the embryo's age to approximate it), it doesn't handle multiples, who are typically delivered earlier, and it knows nothing about your history or your pregnancy. It cannot tell you whether anything is normal. Anything that matters is a conversation with a midwife or doctor, not a web page.