Pregnancy Calculator
Date a pregnancy from whichever anchor you have — last period, due date, conception day, IVF transfer, or ultrasound — and get the due date, weeks pregnant today, trimester, and week-by-week milestones.
Where you are on the 40-week clock
How pregnancy dating works
Clinics count pregnancy from the first day of the last menstrual period (LMP), not from conception — so the “40 weeks” include roughly two weeks before the baby exists. The convention (Naegele’s rule) survives because people know their last period date, not their ovulation date. Every other anchor converts onto the same clock: conception sits near week 2, a day-5 IVF blastocyst goes back 19 days, and a scan report like “8 weeks 3 days” pins the clock directly. After that, trimesters, scan windows, and term dates are plain date arithmetic.
How it’s calculated
Due date = LMP + 280 days (Naegele’s rule), shifted by (cycle − 28) days for other cycle lengths, since ovulation runs ~14 days before the next period. Other modes set the same clock: conception + 266 days; IVF transfer + 263/261/260 days (day-3/5/6 embryos); ultrasound date minus its reported gestational age. Gestational age = days since the adjusted LMP; trimesters switch at 14w0d and 28w0d (ACOG).
Informational only, not medical advice — dating is often revised at the first ultrasound, so confirm your schedule with your provider.
Milestones week by week
Dates are the first day of each pregnancy week on your schedule; screening windows follow typical U.S. prenatal care and vary by clinic.
Worked example
Last period starting March 1, 2026 with a 28-day cycle gives a due date of December 6, 2026. As of July 2, 2026 that pregnancy is 17 weeks, 4 days along — second trimester, with 22 weeks 3 days to go. Conception is estimated around March 15, 2026, and the anatomy-scan week (week 20) begins July 19, 2026.
Common mistakes
- Dating from conception instead of the last period — “6 weeks pregnant” is only about 4 weeks after conception.
- Ignoring cycle length: with a 35-day cycle, ovulation is near day 21 and a plain Naegele estimate runs a week early.
- Mixing week conventions — you are in week 18 while your gestational age reads 17 weeks and some days.
- Planning around the due date itself; only about 4 in 100 babies arrive on that exact day.
Where it is used
- Booking the NT scan (11–13 weeks) and anatomy scan (18–22 weeks) inside their windows.
- Converting an IVF transfer date into a standard OB due date.
- Cross-checking the dating your clinic assigned from an early ultrasound.
- Planning parental leave around the realistic 37–42 week delivery window.
Frequently asked questions
How accurate is the due date?
Only about 4% of babies arrive on the estimated due date, and about 90% within two weeks of it (analysis of NHS birth records, BBC 2015). Treat it as the midpoint of a window — ACOG considers any delivery from 37w0d to 41w6d a normal-term birth.
Which dating method should I trust most?
A first-trimester ultrasound (before 14 weeks) is the most accurate dating method (ACOG Committee Opinion 700); clinics redate to the scan when it disagrees with period-based dating by more than about 5–7 days. IVF dating is more precise still, because the fertilization day is known.
How do irregular or long cycles change the result?
Naegele’s rule assumes ovulation on day 14 of a 28-day cycle, but ovulation tracks the next period (about 14 days before it), so the calculator shifts the schedule by (cycle length − 28) days — a 35-day cycle pushes the due date a week later. For very irregular cycles, use the conception, IVF, or ultrasound mode.
How are IVF due dates calculated?
From the transfer date: + 263 days for a day-3 embryo, + 261 for a day-5 blastocyst, + 260 for a day-6. IVF dating skips the ovulation guesswork because the fertilization day is known exactly.
When do the trimesters change?
This calculator uses the ACOG convention: first trimester through 13w6d, second from 14w0d to 27w6d, third from 28w0d until delivery. Other sources use slightly different cutoffs — the boundaries are conventions, not biology.