Due date calculator
An estimate. Only about one baby in twenty-five is born on its estimated due date. Anything from 37 to 42 weeks is term, but term is four bands rather than one, and full term begins at 39 weeks.
The estimated due date is 40 weeks. 280 days. From the first day of the last menstrual period. Only about 4% of babies arrive on it; term runs from 37 to 42 weeks, so a fortnight either side is entirely normal. Term is not one band, though: full term starts at 39 weeks, and the panel dates all four.
How to find your due date
Two milestones matter more than the due date itself. Twenty-four weeks is the legal threshold of viability in many countries and the point at which neonatal care changes substantially. Thirty-seven weeks is where a birth stops being preterm — the start of early term, not of full term. ACOG separated the two in 2013 precisely because they had been run together: full term begins at 39 0/7 weeks, late term at 41 0/7 and post term at 42 0/7. Babies delivered at 37 and 38 weeks do measurably worse than babies delivered at 39 on breathing, feeding and neonatal admission, and treating 37 weeks as the finish line was costing them that difference. The due date sits at forty and functions mainly as a planning anchor and a reference point for monitoring; its precision is far lower than the specificity of a single named day suggests.
What people use it for
- Announcing a due date
- Planning leave and cover at work
- Booking antenatal appointments
- Telling early term from full term
Questions
Forty weeks from the last period. Anything from 37 to 42 weeks is term, and full term specifically is 39 0/7 to 40 6/7.
No. Thirty-seven weeks is the start of early term, and the point at which a birth is no longer preterm. Full term begins two weeks later, at 39 weeks.
Early term 37 0/7 to 38 6/7, full term 39 0/7 to 40 6/7, late term 41 0/7 to 41 6/7, post term 42 0/7 and beyond. ACOG set them out in Committee Opinion 579.
About 4%. Half of births happen within a week either side.
Yes. A first-trimester dating scan is more accurate than date arithmetic and usually takes precedence.
The commonly used threshold of viability, and a legal boundary in several countries.
Yes, by roughly the number of days your cycle exceeds 28.