Pregnancy Due Dates, Explained
How a due date is calculated: Naegele's rule (LMP plus 280 days), cycle length, conception and IVF dates, early ultrasound, and why few babies arrive on it.
By Udit Agarwal · Updated October 17, 2026
A due date, or estimated date of delivery, is normally calculated by adding 280 days (40 weeks) to the first day of the last menstrual period, a method known as Naegele's rule. A last period that began on 1 January 2026 gives a due date of 8 October 2026. The date marks the middle of a window rather than a deadline: only about 4 percent of babies are born on it, and an early ultrasound scan often refines it.
Named after Franz Naegele, a German obstetrician of the early 1800s, the rule is still the starting point. The American College of Obstetricians and Gynecologists (ACOG) states that by convention the due date is 280 days after the first day of the last menstrual period (LMP), a convention that assumes a regular 28-day cycle.
Counting from the period can seem odd, but its first day is a date most people know, while the day an egg was fertilised usually is not. In a 28-day cycle ovulation comes about 14 days after the period starts, so the 40 weeks include roughly two weeks before conception. Measured from conception, the same pregnancy lasts 266 days, or 38 weeks.
Because 280 days is exactly 40 weeks, the due date always falls on the same day of the week as the first day of the last period:
| First day of last period | Due date (LMP + 280 days) |
|---|---|
| 1 January 2026 (Thursday) | 8 October 2026 (Thursday) |
| 2 March 2026 (Monday) | 7 December 2026 (Monday) |
| 1 May 2026 (Friday) | 5 February 2027 (Friday) |
| 15 June 2026 (Monday) | 22 March 2027 (Monday) |
Older guides give a calendar shortcut: add one year, subtract three months and add seven days. Because months differ in length it is not exact: for a last period on 1 May 2026 it gives 8 February 2027, three days after the true 280-day date of 5 February. Checked across every date, it lands on the right day or up to three days late. To count 280 days exactly by hand, the Date Calculator adds any number of days to a date.
Naegele's rule assumes ovulation on day 14. The part of the cycle after ovulation, before the next period, is fairly steady at about 14 days, and most of the difference between a short and a long cycle comes before ovulation. So in a longer cycle ovulation tends to come later, and a common adjustment moves the due date by the difference between your average cycle and 28 days:
Due date = first day of last period + 280 days + (cycle length − 28 days)
| Average cycle | Shift | Due date for an LMP of 1 January 2026 |
|---|---|---|
| 25 days | 3 days earlier | 5 October 2026 |
| 28 days | None | 8 October 2026 |
| 32 days | 4 days later | 12 October 2026 |
| 35 days | 7 days later | 15 October 2026 |
The adjustment is still an assumption, because the phase after ovulation is not exactly 14 days in everyone and ovulation can move from one cycle to the next. The Ovulation Calculator uses the same 14-day assumption to estimate ovulation and the fertile window in a cycle.
If the date of conception is known, for example from tracking ovulation, the due date is 266 days later: a conception on 10 February 2026 gives 3 November 2026.
For pregnancies from in vitro fertilisation (IVF), the embryo's age at transfer is known exactly, and ACOG advises using it. For a day-5 embryo (a blastocyst) the due date is 261 days after the transfer, and for a day-3 embryo it is 263 days after. Both come to 266 days from the day the egg was fertilised.
| Embryo transferred on 2 March 2026 | Fertilised on | Due date |
|---|---|---|
| Day-5 embryo | 25 February 2026 | 18 November 2026 (transfer + 261) |
| Day-3 embryo | 27 February 2026 | 20 November 2026 (transfer + 263) |
For the same transfer date the day-5 embryo is two days older, so its due date is two days earlier. Weeks are then counted from an equivalent "last period" two weeks before fertilisation, so the dates line up with pregnancies dated from a period.
Pregnancy is counted in completed weeks plus days from the first day of the last period, often written as 17+5 or 17 5/7 weeks for 17 weeks and 5 days. On 17 October 2026, a pregnancy with a last period on 15 June 2026 is 124 days along: 17 weeks and 5 days, in the second trimester, 44 percent of the way to 40 weeks, with 156 days to go until the due date of 22 March 2027.
The first trimester runs to the end of week 13, the second from week 14 to the end of week 27, and the third from week 28. Term covers a five-week span, and since 2013 ACOG has split it into narrower labels:
| Label | Gestational age |
|---|---|
| Preterm | Before 37 weeks 0 days |
| Early term | 37 weeks 0 days to 38 weeks 6 days |
| Full term | 39 weeks 0 days to 40 weeks 6 days |
| Late term | 41 weeks 0 days to 41 weeks 6 days |
| Postterm | 42 weeks 0 days and later |
The due date itself is 40 weeks 0 days, near the middle of full term.
Few. Figures quoted with a 2013 study by researchers at the US National Institute of Environmental Health Sciences, part of the National Institutes of Health (NIH), put the share of women who give birth at exactly 280 days at only about 4 percent, with about 70 percent giving birth within 10 days of the estimated date, even when an ultrasound has been used to set it.
The study itself, published in Human Reproduction, followed 125 pregnancies in which ovulation had been pinned down with daily urine hormone tests, so dating errors could largely be ruled out. The average time from ovulation to birth was 268 days, or 38 weeks and 2 days, and even after six preterm births were excluded, the length of pregnancy varied by as much as 37 days. Natural variation, as well as dating error, spreads births across several weeks.
That is why the due date is best read as the centre of a window. A baby born at any point from 37 weeks 0 days to 41 weeks 6 days is born at term.
ACOG, together with the American Institute of Ultrasound in Medicine and the Society for Maternal-Fetal Medicine, says an ultrasound measurement in the first trimester, up to and including 13 weeks 6 days, is the most accurate way to establish or confirm gestational age. Early embryos grow at very similar rates, so the crown–rump length, measured from the top of the head to the bottom, translates closely into age.
When the scan and the period-based date disagree, ACOG Committee Opinion 700 lists how big the difference has to be at each stage to support changing the due date:
| Gestational age at the scan | Change the due date if the difference is more than |
|---|---|
| Up to 8 weeks 6 days | 5 days |
| 9 weeks 0 days to 13 weeks 6 days | 7 days |
| 14 weeks 0 days to 15 weeks 6 days | 7 days |
| 16 weeks 0 days to 21 weeks 6 days | 10 days |
| 22 weeks 0 days to 27 weeks 6 days | 14 days |
| 28 weeks 0 days and later | 21 days |
The thresholds widen because babies vary more in size as pregnancy goes on, so a later scan dates a pregnancy less precisely. ACOG considers a pregnancy without an ultrasound that confirms or revises the due date before 22 weeks 0 days to be suboptimally dated. If you already have a scan date and the gestational age it measured, the Pregnancy Conception Calculator can work out a due date from those instead of from a period.
The Due Date Calculator works from one of three starting points:
The result shows the due date with its weekday and the number of days to go, or how far past it you are. Below that come today's gestational age in weeks and days, the trimester, the percentage of a 40-week term completed, the estimated conception date and the estimated first day of the last period. Copy buttons give the due date alone or a plain-text summary.
It has limits worth knowing. There is no ultrasound mode, so it cannot use a scan's measurements. Weeks and days are always counted from the period date itself, without the cycle-length shift, so with a 30-day cycle the due date falls at 40 weeks and 2 days on the tool's own count. A cycle length outside 20 to 45 days shows an error rather than a result, and leaving the field blank uses 28. The calculation runs in your browser, and the dates you enter are not sent anywhere.
The result is an estimate from a standard formula and not medical advice; your doctor or midwife sets the due date used for your care.
How is a due date calculated? Add 280 days, or 40 weeks, to the first day of your last menstrual period. A last period that started on 1 January 2026 gives 8 October 2026. If your cycles are longer or shorter than 28 days, a common adjustment moves the date by the difference.
Is the due date counted from conception or from the last period? From the last period, by convention. Conception usually happens about two weeks after a period starts, so the 40 weeks include about two weeks before conception. Counted from conception, the due date is 266 days later.
How accurate is a due date? It is a midpoint, not a prediction of the day. Figures quoted by NIH researchers suggest only about 4 percent of births happen at exactly 280 days and about 70 percent within 10 days of the estimate, even with ultrasound dating.
Can my due date change after an ultrasound? Yes. A first-trimester ultrasound is the most accurate dating method, and ACOG recommends changing a period-based due date when an early scan differs by more than 5 to 7 days, depending on how early the scan is done. Later scans need a bigger difference before the date is changed.
What counts as full term? ACOG defines full term as 39 weeks 0 days to 40 weeks 6 days. Early term is 37 weeks 0 days to 38 weeks 6 days, late term is the 41st week, and postterm starts at 42 weeks.
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