Due date
calculator
The standard estimate clinicians start from: last menstrual period plus 280 days. Enter the date and get the due date, your current week, and the trimester map.
Where the 280 days come from
Naegele's rule, in use since the early 1800s, counts 280 days (40 weeks) from the first day of the last menstrual period — which assumes a 28-day cycle with ovulation on day 14. Pregnancy is dated from the LMP rather than conception because the period date is usually known precisely; it means "week 40" includes about two weeks before conception occurred.
An estimate, honestly
Only about 4–5% of babies arrive on their due date — most come within two weeks either side, and "full term" formally spans 39–40 weeks. Longer or irregular cycles shift real conception later than the rule assumes, which is why a first-trimester ultrasound often refines the date and is considered the most accurate method. Treat this date as the center of a window, not an appointment.
Tracking the earlier stage of the journey? The ovulation calculator maps the fertile window, and the date difference calculator counts down the days.
Due date FAQ
By Naegele's rule: the first day of the last menstrual period plus 280 days (40 weeks). It assumes a 28-day cycle, and a dating ultrasound often refines it.
It's a window, not a deadline — only around 4–5% of births land on the exact date, and most occur within two weeks either side. First-trimester ultrasound dating is the most accurate anchor.
Because the LMP date is usually known exactly while conception rarely is. The convention adds about two weeks before conception into the 40-week count.
Ovulation likely came later, so the rule underestimates — a rough adjustment adds the extra cycle days beyond 28. Your provider will date the pregnancy properly, typically by early ultrasound.