Pregnancy Due Date Calculator

Most due dates use Naegele's rule: take the first day of the last menstrual period and add 280 days, which is 40 weeks. A quick hand method is to add one year, subtract three months, and add seven days; it can land up to three days later than counting exactly 280 days, which is what this calculator does. A last period that started on January 10, 2026 gives a due date of October 17, 2026. The rule assumes a 28-day cycle with ovulation around day 14.

The first day of the last period is a date most people know, while the exact day of ovulation or conception usually is not. Counting from the period gives everyone the same starting line. By this convention a pregnancy is already about 2 weeks along at conception, and the 40 weeks to the due date are about 38 weeks after conception.

A due date is the 40-week point, not a prediction of the day labour starts. Only around 4 to 5 percent of babies arrive on the due date itself, and most births happen within about two weeks either side of it. An early ultrasound is generally considered more accurate than a period date, particularly when cycles are irregular or the period date is uncertain.

Naegele's rule assumes ovulation around day 14 of a 28-day cycle. With a longer cycle, ovulation tends to happen later, so the calculator adds the difference: a 32-day cycle moves the due date 4 days later, and a 25-day cycle moves it 3 days earlier. This assumes the time from ovulation to the next period stays about 14 days, which is a simplification.

Add 266 days to the embryo transfer date, then subtract the embryo's age in days at transfer. A day-5 blastocyst transfer gives the transfer date plus 261 days, a day-3 transfer gives plus 263 days, and a day-6 transfer gives plus 260 days. Because the embryo's age is known exactly, IVF pregnancies are normally dated from the transfer rather than from a period.

Yes. If an early scan measures the pregnancy as noticeably further along or behind the period date, your care provider may move the due date to match the scan. Measurements taken in the first trimester are generally considered the most accurate way to date a pregnancy. Once a due date has been set, it usually stays fixed, even if later scans measure the baby as larger or smaller.

Count the complete weeks since the first day of your last period, adjusted for your cycle length. The calculator shows this as weeks and days, such as 12w 3d. Note that “12 weeks pregnant” means 12 completed weeks, which is the 13th week of pregnancy, so the two ways of counting differ by one.

Using a common convention, the first trimester runs from the start of pregnancy to 13 weeks 6 days, the second from 14 weeks 0 days to 27 weeks 6 days, and the third from 28 weeks 0 days until birth. Some sources draw the lines about a week earlier or later, so treat the boundaries as approximate.

The American College of Obstetricians and Gynecologists uses four labels: early term from 37 weeks 0 days to 38 weeks 6 days, full term from 39 weeks 0 days to 40 weeks 6 days, late term from 41 weeks 0 days to 41 weeks 6 days, and postterm from 42 weeks 0 days. A birth before 37 weeks is preterm.

No. Every calculation runs in your browser. Nothing you enter is stored, sent to a server, or remembered between visits. This tool gives an estimate and general information only; follow the dates your midwife or doctor gives you.