Pregnancy Due Date Calculator
From your last period or conception date — your estimated due date and the trimester map of the next nine months.
Estimate your due date
How due dates are calculated
The standard method is Naegele's rule, in use since the 1800s: due date = first day of your last menstrual period plus 280 days (40 weeks). It assumes a 28-day cycle with ovulation on day 14 — which is why counting from a known conception date instead uses 266 days. Pregnancy is dated from the LMP, which produces the famous oddity that you're "two weeks pregnant" before conception occurs.
What the date actually means
A due date is the center of a probability curve, not an appointment. Only about 4–5% of babies arrive on their due date; the great majority arrive within two weeks either side, and "full term" formally spans 39 weeks 0 days to 40 weeks 6 days. First pregnancies tend to run slightly past the date. Your provider will refine the estimate with a first-trimester ultrasound, which measures the embryo directly and is accurate to within about 5–7 days — when ultrasound and LMP disagree meaningfully, the early ultrasound wins.
The trimester map
First trimester (weeks 1–13): organ formation, the most fatigue, the first prenatal visits. Second (14–27): typically the most comfortable stretch — the anatomy scan lands around week 20. Third (28–40): growth and the home stretch, with more frequent checkups. This calculator marks each boundary so you can plan the practical things — announcements, travel cutoffs, leave paperwork — around real dates. And the obvious but important note: this tool estimates; your OB or midwife dates the pregnancy officially.
Frequently asked questions
My cycles aren't 28 days — is the date still right?
Naegele's rule assumes ovulation on day 14. Longer cycles usually mean later ovulation and a later true due date (roughly: add the days your cycle exceeds 28). The first-trimester ultrasound resolves this properly.
Which is more accurate, LMP or ultrasound?
A first-trimester ultrasound, measured before week 14, is the most accurate dating available (±5–7 days). LMP dating works well with regular cycles and a confidently-remembered date; many providers use both.
When should I tell people?
Entirely personal. Many wait until after the first trimester (around week 13) when miscarriage risk drops substantially, but there is no rule — tell whoever you'd want support from, whenever you wish.
When does the baby actually come?
Statistically: a broad bell curve centered near 40 weeks, with first babies skewing a few days later. About 60% arrive by the due date, and the large majority within 10 days after. Pack the bag by week 36.