Why is a due date counted from the last period rather than intercourse?
This calculator adds 280 days to the first day of the last menstrual period. Choosing the correct starting date is essential to understanding the estimate.
Due date calculator →Use the first day of the period
The input is the day menstrual bleeding began, not the day it ended and not the date of intercourse. For example, adding 280 days to 2026-01-01 gives 2026-10-08. Those 280 days equal 40 weeks. This is an obstetric dating convention; it should not be read as a claim that conception occurred on the first day of the period.
Understand the assumption
The convention relies on a typical cycle assumption and cannot precisely cover differences in ovulation timing, irregular cycles or uncertainty about the last period. This site does not automatically adjust the estimate for cycle length and cannot identify an exact conception date. A more precise-looking calendar output does not remove those limitations.
Use clinical dating for care decisions
Clinicians consider relevant history and ultrasound findings when establishing an estimated due date. The calculator is useful for understanding the calendar arithmetic and making preliminary plans, not for changing a maternity-care schedule independently. The date-addition tool can verify the 280-day calculation, but it adds no medical accuracy. Use the date established by your maternity-care team for decisions about care.
Try the calculators
- Due date calculator
Estimate a due date from the first day of the last period.
- Add days to date
Find the date before or after a number of days.
- Period calculator
Estimate your next period start date from your last period and cycle length.