How to Calculate a Due Date: Naegele's Rule, Conception and Ultrasound

The standard way to calculate a pregnancy due date is to add 280 days (40 weeks) to the first day of the last menstrual period (LMP). This is Naegele’s rule, and it assumes a 28-day cycle with ovulation around day 14. If your last period started January 10, 2026, your estimated due date is October 17, 2026. If you know the date of conception or IVF transfer, or if an early ultrasound gives a different date, the estimate is adjusted, and the ultrasound-based date usually takes priority.

Method 1: from the last menstrual period

The calendar shortcut is add one year, subtract three months, add seven days:

• LMP January 10, 2026 → January 10, 2027 → October 10, 2026 → October 17, 2026

The shortcut can drift a day or two from the exact 280-day count when February falls in the span. For an LMP of March 1, 2026, the shortcut gives December 8, but counting 280 days gives December 6, 2026. Calculators and clinicians use the day count.

Due dates for LMPs on the first of each month (2026)

The pregnancy calculator handles any date and shows trimester milestones.

Adjusting for cycle length

Naegele’s rule assumes ovulation on day 14 of a 28-day cycle. If your cycles are consistently longer or shorter, ovulation likely happens later or earlier, so adjust by the difference:

With a 32-day cycle and the January 10 LMP: 280 + 4 = 284 days, giving October 21, 2026. With irregular cycles, an early ultrasound is especially important.

Method 2: from the conception date

If you know when conception occurred, for example from ovulation tracking:

Conception on January 24, 2026 (14 days after the January 10 LMP) gives October 17, 2026, the same as the LMP method. The conception calculator works backward from a due date, and the ovulation calculator estimates fertile days.

Method 3: after IVF

IVF dates are known precisely, so they give the most reliable non-ultrasound estimate:

Ultrasound dating

Under joint guidance from ACOG, the American Institute of Ultrasound in Medicine and the Society for Maternal-Fetal Medicine, a first-trimester ultrasound measuring crown-rump length is the most accurate method of dating a pregnancy. The due date is changed to match the ultrasound when the difference from the LMP date is greater than:

Ultrasounds later in pregnancy are less accurate for dating because babies grow at different rates. Once a due date is set from the best available information, it generally should not be changed by later scans.

Gestational age and how weeks are counted

Pregnancy weeks are counted from the LMP, about two weeks before conception. So at “6 weeks pregnant,” the embryo is about 4 weeks past fertilization. Gestational age is written as weeks plus days: 24 weeks 6 days is written 24w6d or 24 6/7.

For an LMP of April 20, 2026, on October 11, 2026 the gestational age is 174 days, or 24 weeks 6 days, and the due date is January 25, 2027. The gestational age calculator does this for any date.

Trimesters and term

How many babies arrive on the due date?

Very few, roughly 4% to 5%. Most births occur within about two weeks either side of the due date, and first pregnancies tend to run slightly longer on average. That is why ACOG uses term categories rather than a single day, and why induction decisions consider the full picture, not just the date.

Using your due date

The due date anchors prenatal testing schedules, growth checks and planning for leave. For example, the detailed anatomy ultrasound is usually done at about 18 to 22 weeks, and screening for gestational diabetes is typically done at 24 to 28 weeks, so an accurate date helps those tests happen at the right time. To plan healthy weight gain across the pregnancy, see the pregnancy weight gain calculator, which follows National Academies guidelines based on pre-pregnancy BMI; how BMI is calculated explains that starting number. For counting days and weeks between any two dates, see how to count days between dates.

This guide is for general education and is not medical advice. Your obstetric provider determines your official due date, usually with an early ultrasound. Contact your provider with any questions about your pregnancy.