A due date is estimated by counting 280 days (40 weeks) from the first day of the last menstrual period (LMP). This is known as Naegele's rule, and ACOG describes the average pregnancy as 40 weeks counted from that date. If your last period started on March 1, 2026, the estimated due date is Sunday, December 6, 2026.
The LMP method assumes a 28-day cycle with ovulation around day 14, so the calculator shifts the date for longer or shorter cycles. ACOG's Committee Opinion 700 says the due date should be confirmed or revised by the first accurate ultrasound, and that a pregnancy without an ultrasound before 22 weeks is "suboptimally dated". Your clinician's date takes precedence over this estimate.
How to use the pregnancy due date calculator
- Choose whether you know the first day of your last period or the conception date.
- Enter that date.
- For the LMP method, enter your usual cycle length if it is not 28 days.
- Today’s date fills in automatically; change it to see how far along you will be on another day.
Formula
Gestational age is counted from the LMP, about two weeks before conception, so "6 weeks pregnant" means about 4 weeks after conception. Trimester dates use the common convention of week 14 for the second trimester and week 28 for the third.
Worked example
Last period began March 1, 2026, with a 28-day cycle.
- March 1 + 280 days = December 6, 2026.
- Likely conception: about March 15 (LMP + 14 days).
- On November 10, 2026 the gestational age is 254 days = 36 weeks 2 days, with 26 days to go.
With a 32-day cycle, ovulation is assumed 4 days later, so the due date moves to December 10, 2026.
How accurate is a due date?
A due date is an estimate of the midpoint of the likely delivery window, not a deadline. ACOG says the due date should be set as soon as the last menstrual period, the first accurate ultrasound, or both are known, and that later changes should be reserved for rare circumstances and discussed with the patient. If you conceived by IVF, your clinic will give a date based on the embryo transfer.
To count days between any two dates, use the date difference calculator (days between dates). Staying hydrated matters during pregnancy, but fluid needs change, so ask your clinician rather than relying on the general water intake calculator.
Caveats and assumptions
- An estimate only. Your prenatal care provider’s due date, usually confirmed by ultrasound, takes precedence.
- Cycle-length adjustment assumes ovulation 14 days before the next period; irregular cycles make LMP dating less reliable.
- Not medical advice. Contact a health professional for any concern in pregnancy.
Frequently asked questions
How is a due date calculated?
Add 280 days (40 weeks) to the first day of your last menstrual period, adjusting for cycles longer or shorter than 28 days. From a known conception date, add 266 days.
If my last period was March 1, 2026, when am I due?
December 6, 2026, assuming a 28-day cycle.
Why are pregnancy weeks counted from my period, not conception?
The first day of the last period is usually known while conception is not, so gestational age is counted from it. Conception is typically about two weeks later.
Can my due date change?
An early ultrasound may revise an LMP-based date. ACOG advises that later changes should be rare and discussed with you.
How many months is 40 weeks?
About 9.2 calendar months (280 days ÷ 30.44 days per month on average).
Sources
- ACOG Committee Opinion No. 700 — Methods for Estimating the Due Date — LMP and ultrasound dating; suboptimally dated pregnancies
- ACOG — How long does pregnancy last? — 280 days / 40 weeks from the LMP (confirmed through search results)
- U.S. Naval Observatory — Leap years — Gregorian calendar rules used for date arithmetic
This calculator gives general estimates for healthy adults and is not medical advice, diagnosis or treatment. Equations are averages and individuals differ. Talk to a qualified health professional before changing your diet, exercise or medication, and during pregnancy. See our disclaimer and how we build and check calculators.