Pregnancy Due Date Calculator

How to use the due date calculator

1. In Calculate from, choose Last menstrual period, Conception date, IVF transfer, day-3 embryo or IVF transfer, day-5 embryo.
2. Enter the date. For the last period, it is the First day of your last period, the first day of bleeding.
3. For the last-period method, adjust the Average cycle length (days) if your cycles are usually shorter or longer than 28 days (20 to 45 accepted).
4. Read the Estimated due date and its weekday, the Estimated conception date, your Gestational age today and the Current trimester. The steps show how the date was obtained.
5. The two tables give the date ranges of each trimester and a list of Key dates, with the current one highlighted. Clear empties the date.

Everything is calculated in your browser; the dates you enter are not sent anywhere.

How the due date is calculated

Pregnancy is conventionally counted from the first day of the last menstrual period (LMP), about two weeks before conception. A pregnancy lasts on average 280 days, or 40 weeks, from that date. Naegele’s rule, popularised by the German obstetrician Franz Naegele in the 19th century, is equivalent: add one year, subtract three months and add seven days to the LMP.

Worked example: an LMP of 1 January 2026 gives 1 January 2026 + 280 days = 8 October 2026.

The rule assumes a 28-day cycle with ovulation around day 14. If your cycles last 35 days, ovulation probably happens about a week later, so the tool adds the difference: 35 − 28 = +7 days, giving 15 October 2026. A 21-day cycle moves the date 7 days earlier.

The other methods use the same 280-day timeline:

• Conception date: due date = conception + 266 days (38 weeks), because conception is counted as day 14.
• IVF transfer, day-3 embryo: due date = transfer + 263 days.
• IVF transfer, day-5 embryo (blastocyst): due date = transfer + 261 days.

Weeks and days are written in the usual obstetric notation: 12+3 means 12 weeks and 3 days since the LMP. Trimesters follow the common American College of Obstetricians and Gynecologists convention: the first up to 13+6, the second from 14+0 to 27+6, the third from 28+0 to birth.

How reliable is a due date?

A due date is an estimate, not an appointment. Only about 4 to 5% of babies are born on the exact day. Most are born between 37 and 42 weeks, the range considered term. Cycle length, the date of ovulation and the timing of implantation all vary, and many people are not sure of their LMP.

For that reason, an ultrasound in the first trimester (measuring the crown–rump length) is the most accurate way to date a pregnancy, and your midwife or doctor may change the due date after it. IVF dates are the most precise because the embryo’s age is known. The key dates in the table, such as screening windows, are typical international ranges: follow the schedule given by your own care team, which varies by country.