Pregnancy Due Date
Calculate pregnancy due date
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Working out how to calculate pregnancy due date is easier with Pregnancy Due Date, which answers as you type. Type Last Menstrual Period into Pregnancy Due Date; each keystroke re-runs the math. Pregnancy Due Date displays each intermediate figure as well. Pregnancy Due Date works for one check or many. Nothing typed into Pregnancy Due Date leaves your device or the tab. Pregnancy Due Date is part of the Health collection on CalcProMaster, alongside pregnancy due date calculator by last period, pregnancy week calculator from conception and more. Pregnancy Due Date is designed for real people. Labels stay plain. Pregnancy Due Date answers the next Health question you meet.
Due date from your last period
Pregnancy dating starts from the first day of your last menstrual period rather than from conception, because that is the date people actually know. Naegele's rule adds 280 days — 40 weeks — to that day to give the estimated due date, and it is the same method clinicians use in the first appointment.
Pregnancy is dated from the last period, not from conception, because the LMP is the one date most people can identify. Conception happens about two weeks later, which is why 40 weeks is not 40 weeks of fetal life.
The rule and its arithmetic
Due date = LMP + 280 days. A quick manual version: subtract three months from the LMP and add seven days, then adjust the year. 1 January minus three months is 1 October, plus seven days is 8 October.
Worked example
LMP 1 January 2026
+ 280 days = 8 October 2026
That is 40 weeks exactly.
Conception, in a 28-day cycle, would fall around 15 January.
Why only about 4% of babies arrive on the date
The 280-day figure assumes a 28-day cycle with ovulation on day 14. A woman with a 35-day cycle ovulates around day 21, so the true gestation from LMP is about a week longer and the LMP-based date runs early. A short cycle does the reverse. This is why an early ultrasound is more accurate than any calculation from a date.
What changes the estimate
- Cycle length. Adjust by the difference from 28 days — a 32-day cycle adds four days.
- First-trimester ultrasound — the crown-rump length measurement is the most accurate dating method and usually overrides the LMP date.
- IVF — date from the transfer day and embryo stage rather than the LMP.
- Irregular cycles — ovulation timing is unknown, so the LMP date is a rough placeholder until a scan.
What the date is and is not
It is the estimated date of delivery, the mid-point of a normal distribution. Anything from 37 to 42 weeks is term, and most births fall in the two weeks around the estimate. It is not a deadline and it is not a medical assessment — your midwife or doctor sets the official date after a scan.
Frequently Asked Questions
Why is the due date 280 days after the last period?
Because pregnancy is conventionally dated from the LMP, and 280 days (40 weeks) is the average interval to delivery. Conception occurs about two weeks after that date.
How accurate is the calculation?
For a regular 28-day cycle it is a good estimate, but only about 4% of babies are born on the exact date. An early ultrasound is more accurate and is what clinicians use.
What if my cycle is longer than 28 days?
Add the difference. A 35-day cycle means ovulation around day 21, so the LMP-based date should be about a week later than the calculation gives.
How do I work it out by hand?
Subtract three months from the first day of your last period and add seven days, adjusting the year if needed. 1 January becomes 1 October, then 8 October.
Is 37 weeks full term?
Yes — 37 to 42 weeks is term, with 39–40 weeks considered full term. A baby born at 37 weeks is early term and usually needs no special care.
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