Due Date Calculator – When Is Your Baby Due?

Your 40 weeks, on one line

Pregnancy is counted as 40 weeks from the first day of the last period — about two weeks before conception even happens. The bar shows the three trimesters and where today falls for the dates you entered; the table underneath lists every milestone in between.

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Going deeper

Naegele’s rule, two ways to the same date

The due date on this page is computed by a rule obstetricians have used for about two centuries: add 280 days — forty weeks — to the first day of the last period. The old shortcut, Naegele’s rule, reaches the same date by a different route: take the period date, add a year, subtract three months, add seven days. Here is the estimate this page opens with, worked both ways. The last period began on June 15, 2026. Adding 280 days lands on March 22, 2027. By Naegele’s shortcut: June 15, 2026 plus a year is June 15, 2027; back three months is March 15; plus seven days is March 22, 2027. Two methods, one date — which is reassuring, because it means the arithmetic is not doing anything clever. The 280-day figure is an average across millions of pregnancies, and the rule’s job is to place the centre of a window, not to fix an appointment. Everything else on the page — the trimesters, the countdown, the milestones — hangs off that one anchor date, which is why the page is careful to show its working rather than just the tile.

Because the count starts at the period, “how far along” runs about two weeks ahead of the baby’s actual age. Conception is estimated at day 14 — here June 29, 2026 — and a due date counted from conception adds 266 days instead of 280: June 29, 2026 plus 266 days also lands on March 22, 2027. That is why the three tabs on this page agree when the dates line up, and why the ultrasound tab simply asks how many of the 280 days were already gone on scan day and adds the rest. The count is a convention, not a claim about biology: nobody is pregnant for two weeks before conception, but every provider counts the same way, so the weeks mean the same thing in every clinic. If you ever see a “fetal age” figure that looks two weeks behind the “gestational age” figure, that is the same convention viewed from the other end — subtract a fortnight and the two agree.

Cycle length: the seven-day shift

The 280-day rule quietly assumes a regular 28-day cycle with ovulation on day 14. Ovulation actually happens roughly 14 days before the next period, whenever that falls — so on a longer cycle conception is estimated later than the textbook version, and the due date moves later with it, day for day. Hold the last period at June 15, 2026 and change only the cycle length. Every figure below was produced by the calculator itself.

The relationship is exact and linear: each day of cycle difference moves the due date by one day, because the cycle adjustment is simply your cycle minus 28, added to the 280 days. A 35-day cycle adds seven days; a 21-day cycle subtracts seven. That is also the limit of the correction. It helps if your cycle is consistently long or short, but it cannot help if your cycles vary month to month, if the period date is remembered approximately, if conception followed soon after stopping hormonal contraception, or if periods had only just returned while breastfeeding. Each of those moves conception away from the day-14 assumption in a way no single cycle number can capture. The fixes, in order of reliability, are the ones the deep-dive on this page lists: an early ultrasound, which dates the pregnancy by measurement and sidesteps the calendar entirely; the conception tab, if the day itself is known from tracking or treatment; and the cycle box, which at least corrects for a consistently longer or shorter cycle. If none of those are available, hold the estimate loosely — a provider will date the pregnancy at the first scan, and that date becomes the working one.

The three tabs, and when each wins

Most people know the first day of their last period, so that tab is the standard starting point. But the page offers three anchors, and they are not equal — they differ in what they assume, and the best one is the one whose assumption you can actually trust. The table below runs the same pregnancy through all three, with every figure produced by the calculator itself.

Two things in that table deserve a second look. First, the period tab and the conception tab agree exactly on March 22, 2027 — as they should, because June 29 is day 14 of the June 15 cycle, and 280 days from the period is the same as 266 days from conception. When the tabs disagree, something in the inputs disagrees, and the disagreement is information: it tells you which anchor to trust. Second, the scan example lands on April 1, 2027, ten days after the period-based date — not because the scan is wrong, but because the scan tab’s example inputs describe a different dating: a scan on September 5 saying 10 weeks and 2 days implies a last period around June 25, 2026, not June 15. The scan tab’s own second example makes the mechanics plain: a scan on September 1, 2026 at exactly 12 weeks adds the remaining 196 days to reach March 16, 2027. The reliability order is the one the page states: an early ultrasound first, because it measures rather than counts; the conception date second, if the day itself is known; the cycle-adjusted period date third; and the raw period date last, when cycles are irregular. If a provider gives a different date after a scan, theirs wins — enter the scan result on the third tab and the page rebuilds every milestone around it.

The due date as a window, not a deadline

A due date is the centre of a distribution, not a deadline. Only about 5 in 100 babies are born on the exact day; roughly half arrive in the week either side, and the large majority — around 9 in 10 — arrive within two weeks of it, before or after. A first baby is, on average, slightly more likely to run late than a subsequent one. That spread is biology, not bad maths: the 280-day figure is an average across millions of pregnancies, and any single pregnancy keeps its own schedule within a normal range of about 37 to 42 weeks. The practical way to hold the date is as a planning anchor — for leave dates, for the nursery, for knowing which week you are in — while treating the fortnight around it as the realistic arrival window.

The milestones inside the window are fixed by convention, and the page lists them for the default dates. Trimesters end at 13 weeks plus 6 days and 27 weeks plus 6 days — the convention most providers use — which is why the second trimester starts at week 14 and the third at week 28. For a last period of June 15, 2026, the milestones run: first day of last period Monday, June 15, 2026; estimated conception Monday, June 29, 2026; end of the first trimester Sunday, September 20, 2026; second trimester begins Monday, September 21, 2026; end of the second trimester Sunday, December 27, 2026; third trimester begins Monday, December 28, 2026; due date Monday, March 22, 2027. The trimester and countdown figures are the useful planning tools — the due date is their anchor. And a date change is routine, not alarming: the period-based date is a first estimate, a first-trimester dating scan can move it, usually by a few days, and providers normally adopt the scan date when the two disagree by more than about a week. Later scans rarely change the date, because babies grow at increasingly individual rates as pregnancy goes on, which is why the early scan is the one that dates the pregnancy.

What the countdown actually counts

Beside the due date, the page shows four figures that move every day: how far along you are, which trimester you are in, what percentage of the forty weeks is complete, and how many days remain. All four are computed from the same two dates — the anchor you entered and today — and all four update live as you type. The percentage is simply gestational days divided by 280: on the defaults, viewed on October 24, 2026, the page reports 18 weeks and 5 days, in the second trimester, 46.8% complete, with 149 days to go until March 22, 2027. The arithmetic is 131 days elapsed out of 280, which is 46.8%, and 149 days remaining — the two figures sum to the full 280, as they must.

The same four figures read differently on the other tabs, which is a useful check that the tabs really do rebuild everything. On the scan tab’s defaults — a scan on September 5, 2026 at 10 weeks and 2 days, giving a due date of April 1, 2027 — the same viewing day reports 17 weeks and 2 days, 43.2% complete, and 159 days to go. The pregnancy is the same kind of thing, but the anchor moved, so every derived figure moved with it. And when the due date has passed, the figures do not stop: for a last period of January 5, 2026, the page reports 41 weeks and 5 days, 100% complete, and 12 days past the estimate on October 24, 2026. The percentage caps at 100% because the forty weeks are complete, while the “days to go” figure turns into days past — the page counts forward past the anchor rather than freezing at it. Treat all four figures as what they are: calendar arithmetic on the anchor you chose, useful for planning, and only as good as that anchor. When the anchor changes — most often at the first scan — every figure changes with it, and the new set is the one to plan around.

Twins and multiples: one date, different care

The calculation does not change with the number of babies. The care around it does.

A multiple pregnancy is dated exactly like a singleton pregnancy: add 280 days to the first day of the last menstrual period, adjust for cycle length the same way, and let a first-trimester ultrasound replace the menstrual date on the same thresholds, because the dating rules measure the pregnancy, not the number of fetuses. The estimated due date for twins, triplets, or more is therefore the same 40-week date this calculator gives, and the gestational age on any day is counted the same way, in weeks plus days from the last period or from the scan. Where the counting differs is in what the date is used for: growth is tracked separately for each fetus, screening and monitoring are more frequent, and the plan for birth is made earlier and more explicitly, because multiples on average arrive earlier than singletons and the risks that the care team watches for are different.

The practical consequence is that the due date for a multiple pregnancy should be read as the latest planning point, not as the expected birth day, and the birth plan should be made with the care provider well before it, covering the place of birth, the monitoring schedule, and the circumstances under which an earlier planned birth would be recommended. The countdown on this page counts to the 40-week date either way, and it should be read alongside the provider plan, not instead of it: the provider, with the scan measurements for each fetus, decides the timing, and the calculator date is the anchor the provider plan is counted from. The companion guide works the LMP, ovulation, and conception methods in full, and the IVF guide works the transfer-date calculation, and both apply unchanged to multiples, because the anchor is the same however many babies the pregnancy holds.

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