Due date calculator

What a due date actually is

An estimated due date is not a prediction of the day your baby will be born. It is a reference point: day 280 of a pregnancy counted, by convention, from the first day of your last menstrual period. Everything in maternity care hangs off it — when screening is offered, when a scan is booked, when a birth counts as preterm or post-term — which is why it is worth getting right, and why it is worth understanding how little it promises about the actual day.

The NHS puts the honest version on its own due date calculator: "Pregnancy normally lasts from 37 weeks to 42 weeks from the first day of your last period." That is a five-week window. Research bears it out. A 2013 study in Human Reproduction by Jukic and colleagues opens by noting that only about 4% of women deliver at 280 days, and only 70% deliver within 10 days of their estimated due date — even when that date came from an ultrasound. Their own data, based on pregnancies where the day of ovulation was known from daily hormone measurements, found a median of 268 days from ovulation to birth and a spread of 37 days across term births.

Why the first day of your last period, and not conception

Gestational age is counted from the first day of the last period because that is a date most people can name, whereas the date of conception usually is not. It has one consequence that surprises almost everyone: you are counted as two weeks pregnant before you have conceived. At six weeks pregnant, an embryo has existed for about four weeks. This is bookkeeping, not biology, and it is used consistently by the NHS, ACOG, RCOG and RANZCOG, so a "six week" scan means the same thing everywhere.

Naegele's rule — add 280 days — has been in use for roughly two centuries. Its weakness is that it assumes a regular 28-day cycle with ovulation on day 14. ACOG names the problem directly: the convention "does not account for inaccurate recall of the LMP, irregularities in cycle length, or variability in the timing of ovulation." If your cycles run to 34 days you probably ovulate around day 20, six days later than the rule assumes, and your due date is six days later than the rule gives. That is why this calculator asks for your cycle length and adjusts by cycle − 28, exactly as the NHS calculator does.

Why a scan usually wins

Early ultrasound is the most accurate way to date a pregnancy, because early embryos grow at a remarkably uniform rate. Before about 14 weeks, size really does tell you age. After that, genetics and environment start to pull babies apart in size, and the relationship weakens fast — which is why third-trimester scans are poor at dating and should not be used to move a due date on their own.

The measurement used is crown-rump length: the straight-line distance from the top of the head to the bottom of the torso. Robinson and Fleming published the defining equation for turning that measurement into a gestational age in 1975, after a careful study of where the errors in sonar measurement actually came from. Their equation is still in use fifty years later, and when the INTERGROWTH-21st Project built a modern international standard from thousands of pregnancies across eight countries, it found agreement with Robinson to within about a day across the useful range.

The NHS offers a dating scan at 11 to 14 weeks, and the detailed screening scan between 18 and 21 weeks. If a scan disagrees with your dates, the scan normally wins. ACOG publishes thresholds for when the disagreement is big enough to justify changing the due date: more than 5 days up to 8 weeks and 6 days, more than 7 days from 9+0 to 13+6, more than 10 days from 16+0 to 21+6, more than 14 days from 22+0 to 27+6, and more than 21 days from 28+0 onwards. Below those thresholds the original date stands.

The one exception: IVF

IVF is the only situation where conception is a logged event rather than an inference, so IVF dates are not revised by a scan. ACOG's instruction is specific: a pregnancy from in vitro fertilisation should be dated "using the age of the embryo and the date of transfer" — 261 days from transfer for a day-5 embryo, 263 days for a day-3 embryo. If you conceived through IVF, use the IVF transfer mode here, or the dedicated IVF due date calculator for the full clinic timeline.

Reading the result

Alongside the date, the calculator shows how many weeks and days pregnant you are today, in both plain English ("18 weeks and 2 days") and the notation your maternity notes use ("18+2"). It shows your trimester, the days remaining, and the term bands. Those bands come from ACOG and are worth knowing, because "term" is not one thing: early term is 37+0 to 38+6, full term is 39+0 to 40+6, late term is 41+0 to 41+6, and post term begins at 42+0. Babies born at 39 weeks do measurably better than those born at 37, which is why elective birth without a medical reason is not offered before 39 weeks.

What this cannot do

This is an estimate, not a diagnosis, and it cannot see your pregnancy. It cannot detect twins, which are dated from the larger of the two crown-rump lengths and are rarely carried to 40 weeks. It cannot tell you whether an irregular cycle means you ovulated late this month. And it cannot replace a dating scan — ACOG considers a pregnancy without a dating ultrasound before 22 weeks to be "suboptimally dated". Use this to orient yourself, then let your midwife or doctor set the date that goes in your notes.