How the due date is estimated
Obstetric dating counts pregnancy in weeks from the first day of the last menstrual period, and a typical pregnancy lasts 40 weeks — 280 days — by that count. That figure assumes a 28-day cycle with ovulation on about day 14. If you know the date of conception or embryo transfer instead, the calculator counts forward from that and converts it back to the same 40-week scale, so the "weeks pregnant" figure means the same thing your doctor or midwife uses.
The formulas
From last period: due = LMP + 280 days + (cycle length − 28) From conception: due = conception + 266 days From IVF transfer: due = transfer + 266 days − embryo age (day-5: +261, day-3: +263) Weeks pregnant on a date = (date − (due − 280 days)) / 7
Worked example
Last period started March 1, 2026 with a 28-day cycle: March 1 + 280 days = December 6, 2026. On October 30, 2026, that is 243 days along — 34 weeks 5 days, third trimester.
With a 32-day cycle the date moves 4 days later, to December 10. A day-5 embryo transferred on March 20 gives March 20 + 261 = December 6.
Naegele's rule (add a year, subtract three months, add seven days) gives December 8 for the same LMP; it can differ from the 280-day count by a day or two because months have different lengths.
Limitations
- An early ultrasound is more accurate than any date-based calculation, and your clinician will use it to set the official due date.
- Irregular cycles, recent hormonal contraception or breastfeeding make LMP dating unreliable.
- This tool is for information; it cannot tell you whether a pregnancy is progressing normally.
Sources: American College of Obstetricians and Gynecologists, Committee Opinion 700: Methods for Estimating the Due Date (2017, reaffirmed); ACOG, Committee Opinion 579: Definition of Term Pregnancy.
Frequently asked questions
How accurate is a due date?
It is an estimate. Only about 4% of babies arrive on their due date; most are born within a week or two either side. ACOG recommends confirming or adjusting the date with an ultrasound in the first trimester, which is the most accurate way to date a pregnancy when the last period is uncertain.
Why is pregnancy counted from the last period, not conception?
The first day of the last menstrual period (LMP) is a date most people know, while conception usually is not. Counting from the LMP adds about two weeks before conception, so at conception you are already "two weeks pregnant", and a full pregnancy is 40 weeks (280 days) from the LMP, or 38 weeks (266 days) from conception.
My cycle is longer than 28 days. Does that change the date?
Yes. The 280-day rule assumes ovulation on day 14 of a 28-day cycle. With longer cycles ovulation is later, so the calculator moves the due date by the difference — a 32-day cycle adds 4 days. Ultrasound dating takes over if it disagrees by more than ACOG's thresholds.
How is an IVF due date calculated?
From the embryo transfer date and the embryo's age: add 266 days minus the embryo's age in days. For a day-5 blastocyst that is transfer + 261 days; for a day-3 embryo, transfer + 263 days. This is the method ACOG recommends for pregnancies from assisted reproduction.
What is the difference between early, full and late term?
ACOG and SMFM define early term as 37 weeks 0 days to 38 weeks 6 days, full term as 39 weeks 0 days to 40 weeks 6 days, late term as 41 weeks 0 days to 41 weeks 6 days, and post-term as 42 weeks or more.
This calculator is for general information and screening only and is not medical advice. Talk to a doctor or registered dietitian before making changes based on the result, especially if you are pregnant, under 18, or managing a health condition.