Due Date Calculator

How a due date is estimated

Pregnancy is counted from the first day of your last menstrual period, not from conception. That is about two weeks before an egg is usually fertilized, so at the moment of conception you are already counted as “2 weeks pregnant”. By this convention a pregnancy lasts 280 days, or 40 weeks, and the estimated due date is 280 days after the last period started. This is the method the American College of Obstetricians and Gynecologists (ACOG) describes, and it is often called Naegele’s rule.

The calculator offers three starting points:

• Last period. The standard method. The 280-day rule assumes a regular 28-day cycle with ovulation around day 14. If your cycles are regularly longer or shorter, the calculator moves the due date by the difference, on the assumption that ovulation happened that much later or earlier.
• Conception date. If you know when you conceived, the due date is 266 days (38 weeks) later.
• IVF embryo transfer. The embryo is already a few days old when it is transferred, so fewer days remain: ACOG gives 261 days after a day-5 (blastocyst) transfer and 263 days after a day-3 transfer.

Once you open the page on your own device, the result also shows how many weeks and days pregnant you are today and which trimester that falls in. The milestone table lists the date each stage starts.

Due date formulas

LMP

first day of the last menstrual period

C

usual cycle length in days (28 if unknown)

E

embryo age at transfer: 5 for a blastocyst (261 days), 3 for a day-3 embryo (263 days)

Gestational age — how far along you are — is counted from 280 days before the due date:

Clinicians write gestational age as completed weeks plus days: 37 weeks 2 days is often written 37 2/7 or 37w2d. The stages in the milestone table use these definitions:

The term labels come from ACOG Committee Opinion 579; the trimester boundaries match those used in ACOG Committee Opinion 700. The calculator counts whole days on the calendar, so leap years and month lengths are handled exactly. The traditional pen-and-paper version of Naegele’s rule (add one year, subtract three months, add seven days) can land a day or so away from 280 days, depending on the months involved.

Worked example

Your last period started on Thursday, January 1, 2026, and your cycles are usually 28 days:

1. Due date: January 1, 2026 + 280 days = Thursday, October 8, 2026. Because 280 days is exactly 40 weeks, the due date always falls on the same weekday as the first day of your last period.
2. The first trimester ends on April 8, 2026 (13 weeks 6 days), and the third begins on July 16, 2026.
3. Full term runs from October 1 to October 14, 2026; late term starts October 15 and postterm October 22.
4. On September 19, 2026 you would be 261 days along: 37 weeks 2 days, in the third trimester and early term, with 19 days to the due date.

With a regular 32-day cycle instead, the due date moves 4 days later, to October 12, 2026. And a day-5 blastocyst transferred on January 20, 2026 gives January 20 + 261 days = October 8, 2026 — the same date, because a 28-day cycle, conception on day 14 and a 5-day-old embryo all describe the same pregnancy.

Why an early ultrasound can change your due date

A date worked out from the last period is only as good as the memory and the cycle behind it. ACOG notes that only about half of women accurately recall their last period, and ovulation doesn’t always happen on day 14.

According to ACOG Committee Opinion 700, measuring the embryo or fetus by ultrasound in the first trimester (up to 13 weeks 6 days) is the most accurate way to set or confirm the due date, with an accuracy of about ±5 to 7 days. If the first-trimester scan and the last-period date disagree by more than 5 days before 9 weeks, or by more than 7 days from 9 weeks 0 days to 13 weeks 6 days, the due date is normally changed to match the scan. Scans later in pregnancy are less precise, so a larger difference is needed before the date is changed.

For IVF pregnancies the transfer date is known exactly, so ACOG recommends dating from the transfer rather than from a period or a scan.

What the due date can’t tell you

A due date marks 40 weeks, not the day labor will start. Healthy pregnancies vary in length: in one study that timed ovulation from hormone measurements (Jukic et al., 2013), natural pregnancy length still varied by as much as five weeks. Most babies arrive during the term weeks around the due date rather than on it.

• Irregular cycles. If your cycle length varies a lot from month to month, a last-period date is unreliable and an early scan matters more.
• Recent hormonal contraception or breastfeeding. Ovulation may not follow your usual pattern, so the last-period method may be off.
• Cycle adjustment. Shifting the date by your cycle length is a common approximation that assumes the extra or missing days all fall before ovulation. It can’t know when you actually ovulated.
• Not a health assessment. The calculator doesn’t check how a pregnancy is progressing. Any concern about bleeding, pain or movements needs a midwife or doctor, whatever the date says.

To count the days between any two dates, use the days between dates calculator; to add weeks or months to a date, the date calculator.