Due Date Calculator
Free pregnancy due date calculator. Estimate your due date and current trimester from last menstrual period (LMP) using Naegele's rule — plus week-by-week markers.
Quick answer
Due date = first day of last menstrual period (LMP) + 280 days (Naegele's rule). Pregnancy is divided into trimesters: 1-13 weeks, 14-27 weeks, 28-40 weeks. Only ~5% of babies arrive on the calculated due date — most arrive within 2 weeks of it.
Due Date / Pregnancy Calculator
How it works
Calculates your estimated due date by adding 280 days (40 weeks) to the first day of your last menstrual period — Naegele's rule. Also shows your current week of pregnancy and the trimester you're in.
When to use it
Use this in early pregnancy to get a rough estimate before your first ultrasound, or to track milestones throughout. Doctors will refine the due date with an ultrasound measurement during your first trimester.
Common mistakes
Treating the calculated date as exact. Only about 5% of babies are born on their actual due date — the normal range is anywhere from 37 to 42 weeks. The calculated date is a target, not a prediction.
How the due date calculator works
The standard estimation is Naegele's rule: take the first day of your last menstrual period (LMP), subtract 3 months, and add 7 days to get the due date. This assumes a 28-day cycle and ovulation on day 14. The calculator implements the equivalent (LMP + 280 days) and adjusts for cycle length variations. It also outputs the current pregnancy week, the trimester, and milestone dates (end of first trimester, anatomy scan window, viability, full term).
When to use it
Confirming your provider's due date estimate. Tracking pregnancy progress between OB visits. Planning maternity leave start dates. Knowing when key milestones occur (anatomy scan at ~20 weeks, glucose screening at 24-28 weeks, GBS at 35-37 weeks). Discussing fertility timeline with a partner.
Common mistakes
- Treating the due date as exact. Only about 5% of births occur on the calculated due date. 80% occur within 2 weeks of it. Plan for a 38-42 week range, not a single day.
- Using LMP from an irregular cycle. Naegele's rule assumes 28-day cycles. Cycle lengths over 35 days or under 21 days reduce LMP-based estimates' accuracy. Early ultrasound dating is more reliable in those cases.
- Confusing weeks pregnant with weeks gestation. They're the same — pregnancy is conventionally counted from LMP, not conception. So 'week 6' means 4 weeks post-conception, not 6.
Frequently asked questions
How do you calculate a due date?
Take the first day of your last menstrual period (LMP) and add 280 days (40 weeks). Or use Naegele's rule: LMP minus 3 months plus 7 days. Both produce the same date. The calculator above does this automatically and adjusts for non-standard cycle lengths.
How accurate is the due date?
The calculator gives a probable date, not a guaranteed one. Only about 5% of babies arrive on their due date. 80% are born within 2 weeks of it (38-42 weeks). Early ultrasound dating (especially in the first trimester) is more accurate than LMP-based calculation for irregular cycles.
Can I calculate from conception date instead of LMP?
Yes — add 266 days to conception. But conventional pregnancy timing is measured from LMP, so even providers will refer to weeks based on LMP. Conception-based calculation is mostly used for IVF where the conception date is precisely known.