👶 Pregnancy Due Date Calculator
Calculate estimated childbirth due date and milestone trimester breakdowns instantly.
Estimating a due date, and how much to read into it
An estimated due date is the midpoint of a range, not a deadline. It is calculated as 280 days, or forty weeks, from the first day of the last menstrual period. Fewer than one birth in twenty happens on the date itself, and about four in five happen in the two weeks either side.
Naegele's rule
Franz Naegele formalised the standard method in the early 1800s and it is still what most calculators implement:
EDD = first day of last period + 1 year − 3 months + 7 days
For a last period starting 14 April 2026: add a year to get 14 April 2027, subtract three months to get 14 January 2027, add seven days to get 21 January 2027. That is the same as adding 280 days, with small differences in months of uneven length.
The three ways to date a pregnancy
| Method | Calculation | Typical accuracy |
|---|---|---|
| Last menstrual period | LMP + 280 days | ±1–2 weeks, worse with irregular cycles |
| Known conception or IVF transfer | Conception + 266 days | ±3–5 days, exact for IVF |
| First-trimester ultrasound | Crown-rump length at 8–13 weeks | ±5 days, the clinical standard |
Where LMP and an early scan disagree by more than about seven days, clinical guidelines normally redate the pregnancy to the scan. The scan measures the pregnancy; the LMP method infers it.
Adjusting for cycle length
Naegele's rule assumes a 28-day cycle and ovulation on day 14. For a different cycle length, adjust the due date by the difference:
| Average cycle | Likely ovulation day | Adjustment to EDD |
|---|---|---|
| 24 days | day 10 | 4 days earlier |
| 28 days | day 14 | no change |
| 32 days | day 18 | 4 days later |
| 35 days | day 21 | 7 days later |
IVF dating
IVF is the one case where the dates are known precisely. For a day-5 blastocyst transfer, the due date is the transfer date plus 261 days. For a day-3 embryo transfer, it is the transfer date plus 263 days. No estimation is needed and no adjustment for cycle length applies.
The distribution around the date
For first-time mothers, median gestation is closer to 40 weeks and 5 days than to exactly 40 weeks; subsequent pregnancies tend to run slightly shorter. Research tracking hormonal markers has found natural variation in gestation length of up to five weeks even after accounting for ovulation timing. The spread is biological, not a measurement failure.
Most health systems discuss induction between 41 and 42 weeks, because stillbirth risk rises slowly after 41 weeks. The specific policy varies by country and by individual circumstances, and it is a conversation with your care team rather than a calculation.
A worked example
Last period began 2 June with an average 31-day cycle. Standard Naegele's rule gives 9 March the following year. Adjusting three days later for the longer cycle gives 12 March. A dating scan at 10 weeks then measures a crown-rump length consistent with 14 March, which becomes the official date because it is within the margin where the scan takes precedence.
Every pregnancy is different and these dates are estimates based on population averages. Your midwife, obstetrician or doctor, with an ultrasound, is the authority on your dates. Nothing on this page is medical advice.
Frequently Asked Questions (FAQs)
How accurate is a due date?
About 4% of babies are born on the estimated date and roughly 80% within two weeks either side. It marks the centre of a range covering about five weeks, which is why care teams talk about being term rather than about a single day.
Which is more reliable, my dates or the scan?
A first-trimester ultrasound, measured between 8 and 13 weeks, is accurate to within about five days and takes precedence over LMP dating when the two differ by more than a week. After the first trimester, growth varies enough between babies that scans become poor at dating.
Can the due date change during pregnancy?
It can be revised once, usually after the first dating scan. It is not normally changed on the basis of later scans, because a baby measuring large or small in the third trimester is information about growth rather than about dates.
What is the difference between gestational age and fetal age?
Gestational age is counted from the last period and is what all clinical charts use. Fetal age counts from conception and is about two weeks less. A pregnancy at 20 weeks gestational age contains a fetus about 18 weeks old.
When is it safe to deliver?
Survival improves rapidly from about 24 weeks and outcomes are best from 39 weeks onwards, which is why elective deliveries are generally not scheduled before then without a medical reason. Any decision about timing belongs with your obstetric team.
How do I calculate a due date from an IVF transfer?
Add 261 days to the date of a day-5 blastocyst transfer, or 263 days to a day-3 embryo transfer. IVF dates are exact, so no adjustment for cycle length is needed and ultrasound dating rarely changes them.