What a due date is and what it is not
The estimated due date is the day 280 days — 40 weeks — after the notional start of gestation. That is a convention, not a prediction of the day you will give birth. Gestational age is counted from the first day of the last menstrual period rather than from conception, which means a pregnancy is nominally two weeks old at the moment of conception and roughly four weeks old when a test first turns positive. The convention exists because the last period is a date most people can name and conception is not.
Spontaneous labour clusters around the due date but scatters widely either side of it. Fewer than one birth in twenty happens on the estimated date itself, and around 40 in 100 pregnancies pass the due date without delivering. The date's real job is administrative and clinical: it sets the timing of screening tests, decides whether a baby born early counts as preterm, and determines when a pregnancy is treated as post-term.
Because everything downstream depends on it, the date is fixed once and then generally left alone. Changing it late in pregnancy on the basis of a growth scan is a known error — a third-trimester measurement tells you about the size of the baby, not the age of the pregnancy.
Naegele's rule, cycle length, and the other anchors
Naegele's rule is the arithmetic behind the standard calculation: take the first day of the last menstrual period, subtract three months, add seven days, add a year. That produces almost exactly 280 days — the calendar version drifts by a day or two depending on which months it crosses, which is why this calculator adds 280 days directly rather than shuffling months.
The rule embeds one strong assumption: that ovulation happened on day 14 of a 28-day cycle. Gestation itself is not 280 days from the last period; it is roughly 266 days from conception, and the extra 14 days are the pre-ovulatory phase of the cycle. That distinction is the whole reason cycle length matters. In a longer cycle the follicular phase is longer while the luteal phase stays near 14 days, so ovulation happens later and the due date shifts later by the same amount: EDD = LMP + 280 + (cycle − 28). A 35-day cycle pushes it a week out; a 24-day cycle pulls it four days in.
The other anchors remove the guesswork about ovulation entirely, which is why they use different offsets:
- Known conception or ovulation date. Add 266 days. This is what you use if you tracked ovulation with LH testing or basal body temperature, or if conception followed a single timed insemination.
- IVF day-5 transfer. Add 261 days. A blastocyst transferred on day 5 is already 5 days past fertilisation, and fertilisation itself sits 266 days before the due date, so 266 − 5 = 261.
- IVF day-3 transfer. Add 263 days, by the same logic with a 3-day embryo.
Every one of these lands on the same underlying quantity, which is why the calculator works internally with a single notional “0w 0d” point exactly 280 days before the due date, and expresses gestational age on any date D as 280 − (EDD − D). Whichever anchor you use, the weeks-and-days figure is comparable.
Worked example: last period on 4 May 2026, checked on 3 August
Take a last menstrual period beginning Monday 4 May 2026, with regular 28-day cycles, and work out where things stand on 3 August 2026.
- Apply the cycle adjustment. The cycle is 28 days, so the shift is 28 − 28 = 0 and the 0w 0d point is 4 May itself.
- Add 280 days for the due date. From 4 May to 31 December 2026 is 27 + 30 + 31 + 31 + 30 + 31 + 30 + 31 = 241 days. That leaves 280 − 241 = 39 days into 2027: 31 days takes you to 31 January, and 8 more gives 8 February 2027.
- Count today's gestational age. From 4 May to 3 August: 27 days left in May, plus 30 in June, plus 31 in July, plus 3 in August = 91 days.
- Convert to weeks and days. 91 ÷ 7 = 13 exactly, so 13w 0d.
- Days to go. 280 − 91 = 189 days.
- Trimester. The first trimester runs to 13w 6d, and 13w 0d is inside it, so this is still the first trimester — with six days left in it.
- Share elapsed. 91 / 280 = 0.325, so 32.5% of the nominal 40 weeks.
Cross-check the conception route: conception would be estimated at 4 May + 14 = 18 May, and 18 May + 266 days lands on the same 8 February 2027, because 14 + 266 = 280. Any two anchors that are internally consistent must agree, and if yours do not, one of the two dates is wrong.
How to read the result
Read the weeks-and-days figure first, because that is the number every clinical decision uses. Screening windows are defined in gestational age, not in calendar dates: combined first-trimester screening runs in a window around 11 to 14 weeks, and the detailed anatomy scan is generally offered from about 18 weeks. If your dates are out by a week, the window you are offered moves with them.
Understand the term definitions, because they changed in 2013 and the old single category of “term” is no longer used. ACOG now defines early term as 37w 0d to 38w 6d, full term as 39w 0d to 40w 6d, late term as 41w 0d to 41w 6d, and post-term as 42w 0d and beyond. The distinction matters because outcomes at 37 weeks are measurably different from those at 39, which is why elective delivery before 39 weeks without a medical indication is discouraged.
Treat a due date that has passed calmly and the day count as information rather than a verdict. Around 40 in 100 pregnancies reach the estimated date undelivered, and the negative day count this calculator reports simply says how far past it you are. What changes past 41 weeks is the intensity of monitoring and the conversation about induction, and that is a discussion for your maternity team.
If you are also tracking cycles to conceive again later, or want to sanity-check the ovulation assumption behind the cycle adjustment, the ovulation and fertile window calculator works the same luteal-phase logic in the other direction. For weight tracking through the pregnancy against the Institute of Medicine ranges, see the pregnancy weight gain calculator.
Gestational milestones as day counts
| Milestone | Gestational age | Days from 0w 0d | Days before the due date |
|---|---|---|---|
| Estimated conception (28-day cycle) | 2w 0d | 14 | 266 |
| IVF day-3 transfer equivalent | 2w 3d | 17 | 263 |
| IVF day-5 transfer equivalent | 2w 5d | 19 | 261 |
| End of the first trimester | 13w 6d | 97 | 183 |
| Start of the second trimester | 14w 0d | 98 | 182 |
| Anatomy scan window opens | 18w 0d | 126 | 154 |
| End of the second trimester | 27w 6d | 195 | 85 |
| Start of the third trimester | 28w 0d | 196 | 84 |
| Early term begins | 37w 0d | 259 | 21 |
| Full term begins | 39w 0d | 273 | 7 |
| Estimated due date | 40w 0d | 280 | 0 |
| Late term begins | 41w 0d | 287 | −7 |
| Post-term begins | 42w 0d | 294 | −14 |
Term boundaries follow ACOG's 2013 redefinition. The two columns of day counts always sum to 280, which is a quick way to check any row.
Where menstrual dating goes wrong
- Irregular cycles. The cycle-length adjustment assumes a consistent luteal phase of about 14 days and a follicular phase that absorbs the variation. If your cycles range from 26 to 40 days, no single cycle length describes them and the estimate inherits that uncertainty.
- Bleeding that was not a period. Implantation bleeding or an early-pregnancy bleed mistaken for a light period shifts the estimate a full cycle later, which is one of the commonest reasons a dating scan disagrees with the dates.
- Hormonal contraception recently stopped. The first cycles after stopping are frequently anovulatory or unusually long, so the day-14 ovulation assumption does not hold.
- Conception soon after a previous pregnancy. With no intervening period there is no LMP to date from, and an early scan is the only reliable anchor.
- Changing the date on a late scan. Fetal size varies more and more with age as pregnancy progresses, so a third-trimester measurement is a poor estimator of gestational age. ACOG's dating guidance is built around exactly this: the earlier the scan, the more weight it carries.
- Confusing gestational age with fetal age. Gestational age runs about two weeks ahead of the time since conception. Apps and books occasionally quote embryonic or fetal age instead, which is where a puzzling two-week discrepancy usually comes from.
When an ultrasound date should replace this one
ACOG, the American Institute of Ultrasound in Medicine and the Society for Maternal-Fetal Medicine set out a single dating standard in Committee Opinion 700, Methods for Estimating the Due Date. Its core principle is that the earliest accurate ultrasound gives the most reliable estimate, and that an LMP-derived date should be revised when a scan disagrees with it by more than a defined margin — a margin that widens as the pregnancy advances, because measurement precision falls. If you have had a first-trimester dating scan, the date it produced supersedes anything this calculator gives you, and the whole pregnancy should be dated from it consistently.
Where the 280-day convention came from
Franz Karl Naegele published the rule that bears his name in the early nineteenth century, and it has survived largely because it is easy to do in your head and close enough to be useful. The underlying figure — roughly 266 days from conception — has held up against modern data reasonably well, though studies using precisely timed ovulation have generally found median gestation running a few days longer than the convention implies, and varying with maternal age, parity and the age of the embryo at implantation.
What has changed is the availability of better anchors. Crown-rump length measured between about 7 and 13 weeks estimates gestational age to within a few days, which is far tighter than any menstrual calculation can manage, and IVF gives an exact fertilisation date. Menstrual dating survives because it costs nothing and is available on day one, and because for regular cycles it is usually right to within a week.
Once the baby arrives, the same gestational age keeps mattering for a while. Growth references are indexed by age, and for an infant born preterm the convention is to correct age for prematurity before reading a chart — conventionally until about two years. The child growth percentile calculator handles that side of the arithmetic, and the predicted adult height calculator picks up much later.