Why dating matters
Gestational age drives everything from screening windows to decisions about induction. That is why providers re-date a pregnancy when an early ultrasound disagrees with the last-period estimate by more than about a week.
- • Term is 37–42 weeks; 39–40 weeks is considered full term.
- • Irregular cycles make last-period dating unreliable — get an early scan.
- • Anatomy scans are typically scheduled at 18–22 weeks.
FAQ
How is a due date calculated?
By adding 280 days to the first day of your last menstrual period, adjusted for cycle length. This tool applies that adjustment automatically.
How accurate is a due date?
It is a midpoint, not a deadline. About 80% of births occur within the three weeks either side, and only about 4% happen on the exact date.
What if my cycles are irregular?
Last-period dating becomes unreliable. A first-trimester ultrasound is the accurate method and your provider will use it to set the official date.
How this calculator is built
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Written by Sam Doshi and the RevenueLab editorial team. We don't sell the data feeds this tool is built on.
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