How Your Pregnancy Due Date Is Calculated (and Why It's an Estimate)

The moment a pregnancy is confirmed, the first question is almost always “when is it due?” The answer comes back as a single, specific date — but that date is really the center of a range. Only about one in twenty babies actually arrives on the predicted day. Understanding how the estimate is built makes it far less mysterious, and helps set realistic expectations for those final weeks.

The 40-week count from your last period

By long-standing convention, pregnancy is counted as 40 weeks measured from the first day of your last menstrual period (LMP) — not from conception. This may seem odd, since conception happens roughly two weeks after that, but the LMP is used because it is a date most people can identify reliably, while the exact moment of conception usually cannot be pinned down. The trade-off is that for the first couple of weeks of “pregnancy,” you were not yet pregnant in the biological sense.

Naegele's rule

The classic shortcut for the same calculation is Naegele's rule, named for the German obstetrician who popularized it: take the first day of your last period, add one year, subtract three months, and add seven days. The result is your estimated due date. It is simply a convenient way to add 280 days by hand, and it produces the same answer as the 40-week count.

Why it's only an estimate

The standard calculation assumes a textbook 28-day cycle with ovulation on day 14. Real cycles vary widely. If your cycles run longer or shorter than 28 days, or if ovulation is earlier or later, the LMP-based date can be off by several days. Several factors shift the true date:

• Cycle length. Longer cycles generally push the date later; shorter cycles pull it earlier.
• Ultrasound dating. An early ultrasound measures the embryo directly and is often more accurate than the LMP, so providers may revise the date.
• First versus later pregnancies. First-time pregnancies tend, on average, to go slightly longer.

A full-term delivery is considered anywhere from 37 to 42 weeks. The due date is best thought of as the midpoint of a normal arrival window, not a deadline.

What the trimesters mark

Pregnancy is divided into three trimesters, each with its own broad milestones:

• First trimester (weeks 1–12): major organ development; early symptoms like fatigue and nausea are common.
• Second trimester (weeks 13–27): often the most comfortable stretch; movement is typically felt and anatomy scans take place.
• Third trimester (weeks 28–40): rapid growth and preparation for birth; more frequent prenatal visits.

The fertile window, briefly

For those planning a pregnancy, the fertile window is the handful of days each cycle when conception is possible — generally the five days before ovulation plus ovulation day itself, because sperm can survive several days while the egg is viable for about a day. Cycle-tracking tools estimate this window from your cycle length, but like the due date it is an estimate built on averages.

Using your date wisely

Treat your due date as a planning anchor, not a countdown clock. It is useful for scheduling prenatal care, parental leave, and preparations — but build in flexibility, because babies keep their own schedule. Our pregnancy calculator estimates your due date, current week, and trimester milestones from your last period and cycle length; your healthcare provider, with the benefit of examinations and ultrasounds, will give you the most accurate picture as the pregnancy progresses.