How to use the due date calculator
- Under Date the pregnancy from, pick Last period (LMP), Conception date, IVF 3-day transfer or IVF 5-day transfer.
- Enter the date. For LMP that is the first day of your last period. For IVF it is the day the embryo was transferred.
- With LMP, set Average cycle length if yours is not 28 days. The other methods ignore it.
- Read your due date, current week, trimester and key dates. Everything updates as you change a field.
How the due date is calculated
Every method is converted to an equivalent last period date, and the due date is that date plus 280 days (40 weeks). That is Naegele's rule, the convention described in ACOG Committee Opinion 700. Starting from each date:
- Last period: add 280 days, plus or minus the gap between your cycle and 28 days. A 32-day cycle moves the date 4 days later.
- Conception date: add 266 days (38 weeks).
- IVF transfer: add 263 days for a 3-day embryo or 261 days for a 5-day blastocyst.
Trimesters follow ACOG: the first runs through 13 weeks 6 days, the second from 14 weeks 0 days to 27 weeks 6 days, and the third starts at 28 weeks. ACOG's term definitions are early term (37 weeks), full term (39 weeks), late term (41 weeks) and post-term (42 weeks).
Worked example
With the starting inputs, a last period beginning September 1, 2026 and a 28-day cycle, the due date is Tuesday, June 8, 2027. The estimated conception date is September 15, 2026. The second trimester begins December 8, 2026, the third on March 16, 2027, early term on May 18, 2027, full term on June 1, 2027, and post-term on June 22, 2027. Your current week depends on the day you open the page.
Limits and edge cases
A due date is a midpoint, not a deadline: a 2013 NIEHS study found natural pregnancy length varies by as much as five weeks. Last period dating is weaker with irregular cycles, an uncertain date, or soon after stopping hormonal birth control.
ACOG calls a first-trimester ultrasound the most accurate dating method and redates when it differs from the calculated date by more than 5 days before 9 weeks, or 7 days from 9 to 13 weeks 6 days. This tool cannot redate from a scan, so use your clinician's date. It accepts dates that put you at most 43 weeks along today, or up to a year ahead for planning a transfer, and cycle lengths of 20 to 45 days. To find fertile days first, try the ovulation calculator, and to log cycles, the period tracker.
Frequently asked questions
How do I calculate my due date from my last period?
Add 280 days (40 weeks) to the first day of your last period, then add or subtract the difference between your cycle length and 28 days. The shortcut of adding seven days and counting back three months can land up to three days away from the exact count. A last period beginning September 1, 2026 gives a due date of June 8, 2027.
Is the due date 40 weeks from conception?
No. Pregnancy weeks are counted from the first day of your last period, about two weeks before conception, so the due date falls roughly 38 weeks (266 days) after conception. That is why you count as four weeks pregnant on the day a period is missed.
How accurate is a due date?
It is an estimate, not a deadline. Only about 4 to 5 percent of babies are born on their due date, and most arrive between 37 and 42 weeks. An early ultrasound usually dates a pregnancy better than a remembered last period.
What if my cycles are irregular?
Last period dating is less reliable because ovulation may not have happened on the expected day. Use your best estimate of your average cycle length, or the conception date if you tracked ovulation, and ask your clinician about an early ultrasound.
Sources: ACOG, Methods for Estimating the Due Date (2017); ACOG, Definition of Term Pregnancy (2013).