A due date is the day you reach 40 weeks of pregnancy — not a prediction of when labour will start. Fewer than one baby in twenty arrives on it, and anything from 39 weeks 0 days to 40 weeks 6 days counts as full term. Its real job is to anchor the calendar: it is what every scan, screening test, and clinical decision in the next nine months is measured against.
- What it estimates
- The date you reach 40 weeks 0 days
- From last period
- LMP + 280 days (Naegele's rule)
- From conception
- Conception + 266 days
- From IVF transfer
- Transfer + 266 days − embryo age
- Full term
- 39 weeks 0 days to 40 weeks 6 days
Formula
Naegele's rule, with a cycle-length adjustmentEDD = LMP + 280 days + (cycle length − 28) EDD = conception date + 266 days EDD = transfer date + 266 days − embryo age at transfer
- EDD
- — Estimated date of delivery — 40 weeks 0 days
- LMP
- — First day of the last menstrual period
- cycle length
- — Day 1 of one period to day 1 of the next; 28 is the rule's assumption
- embryo age
- — 3 days for a cleavage-stage transfer, 5 for a blastocyst
The three routes are the same calculation seen from different points. Conception happens about 14 days after the LMP, so 280 − 14 = 266. An IVF embryo was fertilised 3 or 5 days before it was transferred, so its transfer date is that much later again. Whichever you enter, the calculator reduces it to one dating LMP, which is why the modes always agree.
Due Date Calculator examples
| Input | Result | Category |
|---|---|---|
| LMP Jan 15, 2026 · 28-day cycle | Oct 22, 2026 | First trimester |
| LMP Jan 15, 2026 · 35-day cycle | Oct 29, 2026 | First trimester |
| LMP Jan 15, 2026 · 21-day cycle | Oct 15, 2026 | First trimester |
| Conception Feb 1, 2026 | Oct 25, 2026 | First trimester |
| Day-5 blastocyst transferred Mar 3, 2026 | Nov 19, 2026 | First trimester |
| Day-3 embryo transferred Mar 3, 2026 | Nov 21, 2026 | First trimester |
How is a due date calculated?
Naegele's rule adds 280 days — exactly 40 weeks — to the first day of your last menstrual period. Published by Franz Naegele in the early 1800s, it remains the standard calendar method. The shorthand version subtracts three months and adds seven days and a year, which reaches the same answer: an LMP of 19 June 2026 gives 26 March 2027.
The rule assumes a 28-day cycle with ovulation on day 14. If your cycles are consistently longer or shorter, ovulation moves with them and the due date moves by the same number of days. A 35-day cycle ovulates seven days later than a 28-day one, so the due date is seven days later too — an adjustment this calculator makes for you.
Why is pregnancy dated from your last period?
Because the last period is the only event you can reliably date. Conception happens invisibly and its timing is rarely known even to the day. The first day of bleeding, by contrast, is a memorable event, so obstetric convention counts from there.
How is an IVF due date different?
IVF dating is more precise than any other kind, because the date of fertilisation is known exactly rather than inferred. ACOG advises that an IVF pregnancy should be dated from the age of the embryo and the transfer date, and that this dating should not be overridden by a later ultrasound.
| Transfer type | Embryo age | Add to transfer date |
|---|---|---|
| Cleavage-stage (day 3) | 3 days | 263 days |
| Blastocyst (day 5) | 5 days | 261 days |
| Frozen embryo (day 5) | 5 days | 261 days from thaw transfer |
A day-5 blastocyst is two days further along than a day-3 embryo, so a blastocyst transfer is due two days sooner than a cleavage-stage transfer on the same date. For a frozen transfer, what matters is the age the embryo had reached when it was frozen, plus the transfer date — not the date of the original egg retrieval.
How accurate is a due date?
As a prediction of the birthday it is poor; as a reference point it is good enough for everything clinical. Roughly 4% of babies arrive on the estimated date, about half within a week of it, and the great majority within two weeks either side.
Accuracy depends heavily on how the date was set. A crown-rump length measured before 14 weeks is accurate to within about five days; from a remembered LMP the margin is more like a week and a half. ACOG advises redating the pregnancy when an early ultrasound and the LMP disagree by more than the ultrasound's own margin of error.
| Method | Typical margin |
|---|---|
| IVF transfer date | Exact |
| Ultrasound before 14 weeks | ± 5 to 7 days |
| Ultrasound 14–22 weeks | ± 10 to 14 days |
| Last menstrual period, regular cycles | ± 10 days |
| Ultrasound after 28 weeks | ± 21 days or more |
What do the term definitions mean?
In 2013 ACOG and the Society for Maternal-Fetal Medicine replaced the vague label "term" with four narrower ones, after evidence showed that outcomes at 37 weeks are measurably worse than at 39. The distinction matters most when a birth is being scheduled.
| Label | Gestational age |
|---|---|
| Preterm | Before 37 weeks 0 days |
| Early term | 37 weeks 0 days – 38 weeks 6 days |
| Full term | 39 weeks 0 days – 40 weeks 6 days |
| Late term | 41 weeks 0 days – 41 weeks 6 days |
| Postterm | 42 weeks 0 days and beyond |
This is why elective deliveries are not scheduled before 39 weeks without a medical reason, and why most clinicians recommend delivery by 41 to 42 weeks rather than waiting indefinitely.
Frequently asked questions
How accurate is a due date calculator?
It reproduces Naegele's rule exactly, so it is as accurate as the date you feed it. From a confidently remembered last period with regular cycles, expect the real birth within about two weeks either side. An early ultrasound narrows that considerably and takes precedence if the two disagree.
Can my due date change?
Yes. If a first-trimester scan disagrees with your last-period date by more than about a week, your clinician will redate the pregnancy to the scan. This is routine, not a sign of a problem, and the new date is used for all subsequent care.
How do I calculate my due date from conception?
Add 266 days — 38 weeks — to the conception date. That is 14 days fewer than the 280 days counted from your last period, because conception happens roughly two weeks after a period begins. Enter the date in conception mode above and the calculator does it for you.
Does a long cycle change my due date?
Yes. Naegele's rule assumes ovulation on day 14. If your cycles run 35 days, you ovulate about a week later, so your due date is about a week later than the plain rule suggests. The cycle length field above applies exactly that adjustment.
What is the due date for a day-5 embryo transfer?
Add 261 days to the transfer date. The embryo was fertilised five days before transfer, so 266 days from conception becomes 261 from transfer. A day-3 transfer gets 263 days. IVF dating is exact and is not normally overridden by later ultrasound.
What percentage of babies are born on their due date?
About 4%. Roughly half arrive within a week either side, and around 90% within two weeks. Treat the due date as the middle of a fortnight rather than an appointment, and plan leave and childcare around the range rather than the day.
Sources
- Methods for Estimating the Due Date (Committee Opinion 700) — American College of Obstetricians and Gynecologists, 2017
- Definition of Term Pregnancy (Committee Opinion 579) — American College of Obstetricians and Gynecologists, 2013
- Pregnancy — Eunice Kennedy Shriver National Institute of Child Health and Human Development
- Prenatal care: First trimester visits — Mayo Clinic