DocomintPregnancy Due Date Calculator
Estimate a pregnancy due date from the first day of the last menstrual period (Naegele's rule).
Naegele's rule — adding 280 days (40 weeks) to the first day of the last menstrual period — is the standard, quick estimate most pregnancy due dates start from, assuming a typical 28-day cycle; an ultrasound-based estimate from a healthcare provider is more precise and may adjust the date.
Example: A last period starting May 1, 2026 gives an estimated due date of February 4, 2027 (280 days later) — as of today, that's already tracked into a specific week and trimester in the "Current Progress" result.
How it works
- Enter the first day of the last menstrual period
- Press Calculate Due Date
- The estimated due date, current week, and trimester appear instantly, computed on your device
Frequently asked questions
How accurate is Naegele's rule?
It's a widely used estimate, but only about 4-5% of babies are actually born exactly on their calculated due date — it assumes a standard 28-day cycle and ovulation on day 14, which doesn't hold for everyone. An ultrasound-based estimate is generally considered more precise.
Why might my doctor's due date be different from this calculation?
Providers often adjust the estimate based on an early ultrasound measurement of fetal size, which can be more accurate than a last-period-based calculation alone — especially for people with irregular cycles where Naegele's 28-day assumption doesn't hold.
Is my numbers uploaded anywhere?
No — Pregnancy Due Date Calculator runs entirely in your browser using JavaScript/WebAssembly. Your numbers is never sent to a server.