The standard estimate is Naegele's rule: due date = last period + 280 days (40 weeks) — and this tool also works from a known conception date (+266 days) or an IVF embryo transfer (+263 for a day-3 embryo, +261 for day-5). Enter your dates below for your due date, current week, trimester, and milestones — but read it like a clinician does: only about 4–5% of babies arrive on the exact day.
Your due date is the first day of your last period plus 280 days (40 weeks), shifted by one day for each day your cycle differs from 28. The convention assumes ovulation on day 14 — a date most people can recall, while conception usually isn’t. Here it is by hand, two ways:
Exact: Sep 1 + 280 days = June 8, 2027.
Sep 1 − 3 months = June 1 → + 1 year = June 1, 2027 → + 7 days = June 8, 2027.
Across a February the months are short: last period May 1, 2026 → shortcut Feb 8, 2027, exact count Feb 5. Trust the 280-day count.
Know your conception date instead? Conception + 266 days lands on the same date when the 28-day assumption holds.
The due date is the middle of a window. Pregnancy normally lasts 37 to 42 weeks from the last period (NHS), and ACOG names each stretch:
ACOG’s first term category — inside the normal 37–42-week range.
The window the due date sits in; the date itself is its midpoint, not a deadline.
Late term runs to 41w6d; from 42w0d a pregnancy is called post-term (ACOG).
Before 37 weeks is preterm. So “37 weeks” is early term, not full term — a distinction that matters when people talk about scheduling.
As a single day, not very — and that is biology, not bad maths:
Even when the ovulation day is known exactly, healthy pregnancies vary by weeks (Jukic et al., Human Reproduction 2013). Use the date to plan scans, leave and preparation, and the window above for everything else.
Two tools disagreeing on your due date is common, and it almost always comes down to the inputs rather than the maths:
Check the inputs before the tools: same start date, same cycle length, same counting convention. Most tools use the same core rule — 280 days from the last period — so the gap usually lives in what was fed in.
A first-trimester scan measuring crown–rump length is the most accurate dating method. Whether it replaces your period-based date depends on the size of the gap:
The two due dates are compared at the earliest ultrasound; once the date is set, it is changed again only in rare cases.
From 9 weeks to 15 weeks 6 days the threshold is 7 days.
Later in pregnancy the thresholds widen — all five are in the table below (ACOG).
The allowed gap widens as pregnancy goes on, because a scan dates a pregnancy less precisely the later it is done:
| Weeks at first scan | Measured by | Redate if the gap is |
|---|---|---|
| Up to 8w6d | Crown–rump length | More than 5 days |
| 9w0d – 15w6d | Crown–rump length to 13w6d, then head, abdomen and thigh-bone measurements | More than 7 days |
| 16w0d – 21w6d | Head, abdomen and thigh-bone measurements | More than 10 days |
| 22w0d – 27w6d | Head, abdomen and thigh-bone measurements | More than 14 days |
| 28w0d and later | Head, abdomen and thigh-bone measurements | More than 21 days — and only with care (see below) |
A 3-day gap at 8 weeks is inside the 5-day tolerance; your period-based date stands.
A small baby measured in the third trimester may be growth-restricted rather than younger, so ACOG says decisions from a third-trimester scan alone need the whole clinical picture and close follow-up.
Growth scans after the date is set usually track size, not age; if a clinician does change the date again, ACOG asks for it to be discussed with you and documented.
With an embryo transfer the embryo’s age is known, so the due date comes from the transfer date (ACOG), not from scan redating.
| Trimester | Weeks | Demo dates | Notes |
|---|---|---|---|
| First | Week 1 – 13+6 | Sep 1 – Dec 7 | Organ formation; dating ultrasound is most accurate in this window |
| Second | Week 14 – 27+6 | Dec 8 – Mar 15 | Anatomy scan typically at 18–22 weeks |
| Third | Week 28 – birth | Mar 16 onward | Full term from week 39; due date at week 40 |
Working backwards instead? The conception calculator estimates when conception likely happened. Not yet sure you’re pregnant? Start with the pregnancy test calculator.
Naegele's rule: last period + 280 days. With a non-28-day cycle, shift by (cycle length − 28) days.
Only about 4–5% of babies arrive on the exact date; most come within roughly two weeks either side. It's a planning anchor, not a deadline.
Adjust by (cycle length − 28): a 35-day cycle moves the due date about 7 days later. The calculator applies this to the due date automatically. It does not move the week count: today’s week and the milestone dates still count from your period date, so with a 35-day cycle the due date lands at 41+0 on that count rather than 40+0. A first accurate scan usually settles both — see why pregnancy is counted from your last period.
Yes, by set rules, and usually only once. Your clinician compares the earliest ultrasound with the LMP estimate and redates only past set gaps: more than 5 days up to 8w6d, 7 days at 9–15w6d, 10 at 16–21w6d, 14 at 22–27w6d, and 21 days at 28 weeks or later (ACOG Committee Opinion No. 700). After that, later changes are rare — a bigger- or smaller-than-expected baby on a growth scan is usually a growth question, not a new due date — and a late first scan is read with extra caution. IVF pregnancies keep the transfer-based date.
Not quite. ACOG's term bands: before 37 weeks is preterm; 37–38+6 is early term; 39–40+6 is full term (your due date sits in this band); 41–41+6 is late term; 42 and beyond is postterm. "Term" loosely covers 37–42, but full term specifically starts at 39 weeks.
Pregnancy counted from the last period, so it runs about 2 weeks ahead of the time since conception: "8 weeks pregnant" ≈ 6 weeks since conception. Why it is counted this way, and how to convert test readouts and scan dates: why pregnancy is counted from your last period.
Conception + 266 days — equivalent to LMP + 280 when the 28-day assumption holds.
First: through 13+6. Second: 14–27+6. Third: from 28. Full term from week 39.
LMP dating becomes unreliable because the ovulation assumption breaks — a first-trimester ultrasound is the dependable way to date the pregnancy.
Digital tests with a weeks indicator (like Clearblue) count from conception, while clinicians count from the last period — about 2 weeks earlier. So a “1–2” reading means roughly 3–4 weeks pregnant, “2–3” means 4–5 weeks. Enter your last period above to see the clinical week.
IVF dating is more precise because the embryo’s age is known exactly: due date = transfer day + (266 − embryo age in days). A day-5 blastocyst transfer means +261 days; a day-3 transfer means +263 days. The transfer date replaces the LMP entirely — no cycle-length adjustment applies. The result also shows an equivalent LMP: the period date that would give the same due date. It is a dating reference, not a period you had — for a clinic or form field, use the dates your clinic recorded.
Nothing — switch to the IVF/embryo-transfer mode above and enter the transfer date plus embryo age (day-3 or day-5). IVF dating needs no LMP at all. Clinics sometimes give a "nominal LMP" for their records; this calculator shows an equivalent last-period date as a dating reference, which may not match the date your clinic uses — go by your clinic’s records for any form.
Apps assume ovulation on day 14 of a 28-day cycle; your doctor has your actual scan. When a first-trimester ultrasound and the LMP estimate disagree by more than the threshold for that week (over 5 days up to 8w6d, over 7 days from 9w0d), ACOG uses the scan date. Treat the app date as the placeholder until the dating scan. IVF is different: the transfer date sets the due date.
LMP-based dating needs a date you can identify. If you can’t, skip the estimate and rely on a first-trimester ultrasound — crown-rump length dating is accurate to about ±5–7 days and doesn’t depend on memory. Testing first? The pregnancy test calculator covers timing.
Pregnancy is 40 weeks but only about 9 calendar months + 1 week — months aren’t 4 weeks, so the two never line up cleanly. Rough mapping: 4 weeks = month 1, 8 weeks = month 2 … 40 weeks = end of month 9. Clinicians use weeks precisely because months are ambiguous.