Pregnancy Due Date Calculator
Estimate when your baby is due from the first day of your last period, adjusted for your cycle length, and see your current gestational age and trimester.
Last updated: June 20, 2026 · Medically reviewed by TG we-Calculate Editorial TeamFirst day of last period (LMP)
days
Today's date
Thursday, October 8, 2026
How is a pregnancy due date calculated?
A due date is estimated with Naegele’s rule: add 280 days (40 weeks) to the first day of your last menstrual period, then adjust by your cycle length minus 28 days. Gestational age is counted from that same first day, so the count includes the roughly two weeks before conception.
Formula
How this is calculated
The calculator takes two inputs: the first day of your last menstrual period (LMP) and your average cycle length in days. It starts from Naegele’s rule, which adds 280 days (40 weeks) to the LMP. That figure assumes a textbook 28-day cycle in which ovulation happens on day 14, so conception is treated as roughly LMP + 14 days. All dates are handled as whole calendar days at local midnight, so there are no fractions of a day.
Because real cycles vary, the tool corrects for them with a single adjustment of (cycleLength − 28) days: a longer cycle shifts ovulation—and therefore the due date and estimated conception—later, while a shorter cycle moves both earlier. Gestational age is measured separately, counted in completed weeks and days from the LMP (not from conception), which is why a "40-week" pregnancy includes the ~2 weeks before fertilisation. Trimesters follow obstetric convention: weeks 0–13, 14–27, and 28+.
The key limitation is that this is an estimate built on assumed regular, day-14 ovulation. It is less reliable with irregular cycles or an uncertain LMP, and only about 4% of births land on the exact date—a full-term delivery is normal anywhere from 37 to 42 weeks. An early dating ultrasound is more accurate.
Examples
| Input | Result |
|---|---|
| LMP 2026-01-01, 28-day cycle | Due 2026-10-08 (280 days later) |
| LMP 2026-01-01, 35-day cycle | Due 2026-10-15 (+7 days for the longer cycle) |
| LMP 2026-01-01, 21-day cycle | Due 2026-10-01 (−7 days for the shorter cycle) |
About this calculator
The pregnancy due date calculator uses Naegele’s rule, the standard obstetric method for estimating a delivery date. It adds 280 days (40 weeks) to the first day of your last menstrual period (LMP). Because the rule assumes a textbook 28-day cycle with ovulation on day 14, the calculator shifts the estimate by (cycleLength − 28) days so a longer or shorter cycle moves your due date later or earlier.
Gestational age is counted in completed weeks and days from the first day of the LMP — not from conception — which is why a pregnancy is described as "40 weeks" even though fertilisation occurs about two weeks in. The first trimester runs through week 13, the second through week 27, and the third from week 28 until birth. Conception is estimated at roughly LMP + 14 days (also adjusted for cycle length).
A due date is an estimate, not a deadline: only about 4% of babies arrive on the exact date, and a normal full-term delivery can occur any time between 37 and 42 weeks. An early dating ultrasound is more accurate than LMP-based dating, especially when cycles are irregular or the LMP is uncertain.
Frequently asked questions
LMP-based dates are estimates and assume regular ovulation on cycle day 14. Only about 4% of babies are born on the exact due date, and a full-term birth can happen any time from 37 to 42 weeks. An early ultrasound gives a more precise date.
Naegele’s rule assumes a 28-day cycle with ovulation on day 14. If your cycle is longer or shorter, you ovulate later or earlier, so the calculator shifts the due date by your cycle length minus 28 days.
Gestational age is how far along the pregnancy is, counted in weeks and days from the first day of your last period — not from conception. That is why 40 weeks gestation includes the roughly two weeks before fertilisation.
Also known as
Sources & references
TG we-Calculate Editorial Team. (2026). Pregnancy Due Date Calculator [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/due-date-calculator
TG we-Calculate Editorial Team. "Pregnancy Due Date Calculator." TG we-Calculate. 2026. https://we-calculate.com/calculator/due-date-calculator.
TG we-Calculate Editorial Team, "Pregnancy Due Date Calculator," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/due-date-calculator
@misc{wecalculate_due_date_calculator, title = {Pregnancy Due Date Calculator}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/due-date-calculator}}, year = {2026}, note = {TG we-Calculate} }
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