Pregnancy Due Date Calculator

Why pregnancy is dated from before conception

The standard due date calculation counts 280 days from the first day of your last menstrual period, not from conception — which typically happens roughly two weeks later. This convention dates back to the 19th-century obstetrician Franz Naegele, and it persists because the start of the last period is a date most people can state with confidence, while the exact date of conception usually is not.

Naegele's rule and its known bias

Naegele's rule assumes a textbook 28-day cycle with ovulation on day 14, which does not describe every cycle. Shorter or longer cycles shift the real conception date without shifting the calculation, which is one reason research suggests the rule runs slightly early for a meaningful share of pregnancies. An early ultrasound measuring the fetus directly generally provides a more accurate estimate than last-period dating alone, particularly for people with irregular cycles.

Why almost no one arrives exactly on the due date

A due date is the midpoint of a normal range, not a prediction. Full term is defined as anywhere from 37 to 42 weeks, and only a small minority of births land on the calculated date itself. The date is best read as the centre of several weeks of normal variation rather than a deadline — a first-time pregnancy in particular skews slightly later than the average.

What each trimester boundary actually marks

The three trimesters are a convenience for discussion, not a biological switch that flips on a specific day. Roughly: the first trimester (weeks 1–12) covers the period of highest miscarriage risk and the formation of major organs; the second (13–27) is typically when energy improves and anatomy scans happen; the third (28 onward) is dominated by growth and preparation for birth. The boundaries are approximate by convention, not by anything that happens biologically at week 13 or 27 specifically.

Why this is a planning tool, not medical guidance

This calculator applies a public, decades-old formula to a date you provide — it has no information about your specific pregnancy, your cycle length, or anything an ultrasound would show. Its output is useful for planning and general orientation. Anything about the health of a specific pregnancy is a question for the clinician actually monitoring it, who has information this page cannot.