Pregnancy week calculator
The due date is counted 280 days — 40 weeks — from the start of the last period. It is a statistical average, not a promise: term birth spans weeks 37 to 42, and an ultrasound dates the pregnancy more precisely.
- Estimated due date
- 11/06/2027
- Trimester
- 1
- Days remaining
- 280
Basis of the calculation
The due date uses Naegele's rule: 280 days, or 40 weeks, from the first day of the last period. Actual birth dates often differ by a couple of weeks.
How accurate is the calculated due date?
Only about 4–5% of babies arrive exactly on the due date; most come within two weeks either side. The calculation assumes a 28-day cycle — a longer or shorter cycle shifts the date. A first-trimester ultrasound usually pins the date to within a few days.
How are pregnancy weeks counted?
Weeks count from the last period's start, not from conception — so during the first "week of pregnancy" there is no pregnancy yet. Conception typically happens late in week two. When a doctor says "week 12", it means 11 full weeks plus the days under way.
When do the trimesters start and end?
The most common split: first trimester weeks 1–13, second 14–27, third from week 28 to birth. The boundaries are conventions and vary slightly by source, so a book and an app disagreeing by a week is normal. What matters clinically is the exact week, not the trimester label.
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