How to Calculate Your Due Date

Your estimated due date anchors prenatal appointments and milestone tracking. Here are the three ways it is calculated (from your last period, from a known conception date, and by early ultrasound), with a worked example. This article is educational and not a substitute for advice from your own healthcare provider.

How Due Dates Are Estimated

The most widely used method for estimating a due date is called Naegele's rule, named after the German obstetrician who popularized it in the 19th century. The rule is built on a simple idea: a typical pregnancy lasts about 280 days, or 40 weeks, counted from the first day of your last menstrual period (LMP).

The formula:

Estimated due date = first day of last menstrual period + 280 days

The traditional shorthand: take the first day of your LMP, subtract three months, add seven days, and add one year. It lands on the same date or within a day or two of it, depending on month lengths.

Counting from your last period works because most people know when it started, while few know the day they conceived. (The pregnancy week calculator turns the same count into your current week.) The method assumes a regular 28-day cycle with ovulation around day 14. If your cycles are longer, shorter, or irregular, the LMP-based estimate may be slightly off, which is one reason providers often confirm dating with an ultrasound.

A Worked Example

Imagine your last menstrual period began on January 10, 2026. To find your estimated due date using Naegele's rule, you add 280 days.

Start date: January 10, 2026
Add 280 days (40 weeks)
Estimated due date: October 17, 2026

Check with the shorthand: January 10, 2026 minus three months is October 10, 2025; plus seven days is October 17, 2025; plus one year is October 17, 2026.

Counting 280 days by hand is easy to get wrong across months of different lengths and leap years. The due date calculator takes how many days ago your last period started (or, if you know it, your conception date), plus your cycle length and luteal-phase length, and returns the days until your due date and your current week. From an LMP it counts cycle length minus luteal phase (14 days in a standard cycle) to ovulation and adds 266 days; from a conception date it adds 266 days directly.

Conception Date and Ultrasound Dating Methods

Naegele's rule isn't the only way to estimate a due date. Two other methods are commonly used, often to refine or confirm the LMP-based estimate.

Conception date method. If you know the exact date of conception, perhaps because you were tracking ovulation, used fertility treatments, or had a single, well-timed intercourse, your provider can count forward 266 days (38 weeks) from that date. This works because conception typically occurs about two weeks after the first day of your last period, so the conception-based count is roughly 14 days shorter than the LMP-based count.

Ultrasound dating. An early ultrasound measures the embryo or fetus (the crown-rump length) and compares it with expected growth. Early in pregnancy fetuses grow at very similar rates, so ACOG considers a first-trimester ultrasound (up to 13 weeks 6 days) the most accurate way to establish or confirm gestational age, accurate to about ±5–7 days. If it differs enough from your LMP-based date, your provider will adjust the due date. Later ultrasounds become less reliable for dating because growth rates vary more between individual babies as pregnancy progresses.

Why Few Babies Arrive on the Due Date Itself

Only a small share of babies are born on the estimated date. That doesn't mean the calculation is wrong: the due date is the midpoint of a natural range.

ACOG classifies births at 37 weeks 0 days to 38 weeks 6 days as early term, 39 weeks 0 days to 40 weeks 6 days as full term, 41 weeks as late term, and 42 weeks or more as postterm. Even when ovulation was pinpointed with hormone tests, a 2013 study in Human Reproduction found that natural pregnancy length varied across a 37-day range. Cycle length, genetics and natural variation all play a part.

Trimester Milestones

Your 40 weeks are divided into three trimesters, each marking a distinct stage of development.

Trimester boundaries are drawn slightly differently by different sources; ACOG uses up to 13 weeks 6 days for the first trimester and 14 weeks 0 days to 27 weeks 6 days for the second.

First trimester (to about week 13). Major organs and body systems begin forming and cardiac activity appears. Many people have early symptoms such as fatigue and nausea.

Second trimester (about weeks 14–27). Energy often returns and early symptoms ease. Many people feel the first movements, and an anatomy ultrasound, usually at 18–22 weeks, checks development and can show the baby's sex if you want to know.

Third trimester (weeks 28-40). The baby gains weight rapidly, the lungs mature, and it settles into a head-down position for birth. Prenatal visits become more frequent as you approach delivery.

When to Talk to a Provider

Your due date is a starting point, not a substitute for professional care. You should reach out to your healthcare provider in several situations: to confirm your pregnancy and establish accurate dating early on, if your menstrual cycles are irregular and you're unsure of your LMP, or if an ultrasound estimate differs from your calculated date. Always contact your provider promptly if you experience warning signs such as significant bleeding, severe pain, reduced fetal movement in later pregnancy, or signs of preterm labor before 37 weeks. They can interpret your individual situation in a way no calculator can.