Pregnancy due date calculator
Estimated due date from your last period (any cycle length), conception date or IVF transfer, with weeks pregnant, trimester and key dates.
Your estimated dates
Enter the first day of your last menstrual period (LMP) and your usual cycle length to estimate your due date. The calculator uses Naegele’s rule, the method clinicians use: due date = LMP + 280 days (40 weeks), adjusted by the number of days your cycle is longer or shorter than 28. For a period that started on 1 January 2026 and a 28-day cycle, the estimated due date is 8 October 2026; with a 32-day cycle it moves 4 days later, to 12 October.
If you know the conception date, the due date is 266 days (38 weeks) later. After IVF it is 263 days after a day-3 embryo transfer and 261 days after a day-5 (blastocyst) transfer. The result shows how many weeks and days pregnant you are today or on any date you choose (counted from the LMP-equivalent date, 280 days before the due date), your trimester, and key dates: the end of the first trimester at 13 weeks 6 days, the usual window for the mid-pregnancy anatomy scan at 18 to 22 weeks, early term from 37 weeks, full term from 39 weeks, the due date at 40 weeks and 42 weeks.
These dates are estimates. Only about 4 to 5% of babies arrive on their due date, and most are born between 37 and 42 weeks. A dating ultrasound in the first trimester is more accurate than dates from a period, particularly if your cycles are irregular or you are not sure when your last period began; if your clinician gives you a due date from a scan, use theirs. Please talk to a doctor or midwife about any concern. The method follows the American College of Obstetricians and Gynecologists guidance on estimating the due date. How we calculate this: see the formula table in the result. Last reviewed: 2026-09-21.
How to use it
- Choose how to date the pregnancy: last period, conception date or IVF transfer.
- Enter the date. For the last-period method, also enter your average cycle length (28 if you are not sure).
- Leave “Show weeks pregnant on” empty for today, or choose another date.
- Read the estimated due date, weeks pregnant, trimester and the table of key dates; the “How we calculate this” table shows the formulas used.
Frequently asked questions
How is the due date calculated from the last period?
By Naegele’s rule: add 280 days (40 weeks) to the first day of your last menstrual period. It assumes a 28-day cycle with ovulation around day 14, so for a longer or shorter cycle the calculator moves the date by the difference: a 31-day cycle adds 3 days.
Why is pregnancy counted from the last period and not from conception?
Because the first day of a period is usually known and conception is not. By convention, gestational age starts about two weeks before conception, so at conception you are counted as 2 weeks pregnant. That is why a pregnancy is “40 weeks” although the baby develops for about 38.
How accurate is the due date?
It is an estimate. A first-trimester ultrasound measuring the embryo is the most accurate way to date a pregnancy, and clinicians change the due date when the scan differs from the period-based date by more than a few days. IVF dates are the most precise because the embryo’s age is known.
What do early term, full term and post-term mean?
Early term is 37 weeks 0 days to 38 weeks 6 days, full term 39 weeks 0 days to 40 weeks 6 days, late term 41 weeks 0 days to 41 weeks 6 days, and post-term 42 weeks or more. Your care team will discuss what happens as you approach and pass your due date.
My cycles are irregular. Can I still use this?
You can, but the date will be less reliable, because ovulation may not have happened when a regular cycle would predict. Ask your doctor or midwife about a dating scan. See a doctor promptly if you have bleeding, pain or any other concern.
Is my information stored?
No. The calculation runs in your browser and the dates you enter are not sent to a server or saved.
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