How a Pregnancy Due Date Is Calculated

Gestational age is conventionally counted from the first day of the last menstrual period (LMP), not from conception. Conception typically happens about two weeks after the LMP in a 28-day cycle, so a pregnancy is counted as 40 weeks long even though the baby has been developing for about 38 weeks. This convention exists because the LMP is usually known, while the date of ovulation or conception often is not.

Naegele’s rule estimates the due date (estimated date of delivery, EDD) as 280 days after the first day of the LMP, which is exactly 40 weeks. A traditional way to write it is: add one year, subtract three months and add seven days. Both give the same date.

Worked example: LMP on 10 January 2026. Adding 280 days gives 17 October 2026 (a Saturday). Using the “subtract three months, add seven days” method gives 10 October + 7 days = 17 October, the same date.

Naegele’s rule assumes a 28-day cycle with ovulation near day 14. If your cycles are consistently shorter or longer, ovulation tends to occur earlier or later, and the due date shifts by the difference:

adjusted EDD = LMP + 280 days + (cycle length − 28)

For the same LMP of 10 January 2026:

• A 32-day cycle adds 4 days, giving 21 October 2026.
• A 26-day cycle subtracts 2 days, giving 15 October 2026.

Irregular cycles make the LMP method less reliable, which is one reason clinicians often confirm dating with an ultrasound.

In early pregnancy, ultrasound measurements are used to estimate gestational age, and a clinician may adjust the due date if the scan differs from the LMP date by more than a set margin. Early scans are generally considered more accurate for dating than later ones because early fetal growth is relatively uniform. If your clinician gives a due date that differs from a calculator, follow your clinician’s date.

Gestational age is written as weeks plus days. A pregnancy at 23 weeks 0 days on 20 June 2026 with an LMP of 10 January 2026 is (20 June − 10 January) = 161 days = 23 weeks exactly. To convert days to weeks and days, divide by 7: the quotient is weeks and the remainder is days.

Pregnancy is often divided into trimesters. The first trimester runs through 13 weeks 6 days, the second from 14 weeks 0 days to 27 weeks 6 days, and the third from 28 weeks onward. Terms such as early term, full term and late term describe the weeks at which birth occurs, and are used by clinicians for care planning; your care team will explain the definitions they use.

Only a small proportion of babies are born on their exact due date. Many are born in the weeks before or after. It is more accurate to think of the EDD as the centre of a window. Your clinician will watch for signs that require action if the pregnancy extends beyond the expected range.

• The date you reach each week: LMP plus (weeks × 7) days. At 14 weeks that is 18 April 2026 for the example LMP, at 28 weeks 25 July 2026, and at 37 weeks 26 September 2026.
• Days remaining: due date minus today.
• Conception window: approximately LMP plus 14 days in a 28-day cycle, with a margin.

Weight guidance in pregnancy depends on the starting body mass index and is set by clinicians; calculators give general ranges only.

• Counting from conception instead of the LMP.
• Using the wrong first day: day 1 is the first day of bleeding, not the last.
• Ignoring cycle length when it differs notably from 28 days.
• Treating the EDD as an exact date.
• Overriding a clinician’s ultrasound-based date with a calculator.

Enter the first day of your last period in the Pregnancy Due Date calculator to get the EDD. The Pregnancy Week Tracker shows how far along you are on a given date. These are educational tools and do not replace prenatal care.