DocomintPregnancy Due Date Calculator
Estimate a pregnancy due date from the first day of the last menstrual period (Naegele's rule).
What is a Pregnancy Due Date Calculator?
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.
How to use the Pregnancy Due Date Calculator
- 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
Formula
Due Date = First Day of Last Menstrual Period + 280 days (40 weeks)
Naegele's rule assumes a standard 28-day cycle with ovulation on day 14 — the standard quick estimate most due dates start from before any ultrasound adjustment.
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.
Common mistakes
- Using it with a cycle that isn't close to 28 days — the rule's 280-day offset assumes that specific cycle length, and it drifts for shorter or longer cycles.
- Treating the estimated date as exact rather than the center of a real range most pregnancies fall within.
- Confusing "weeks pregnant" (current progress) with "trimester" (a broader three-part grouping of those weeks) when reading the result.
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.
What if my cycle isn't 28 days?
Naegele's rule assumes a 28-day cycle, so the estimate drifts for longer or shorter cycles — a healthcare provider typically adjusts the estimate based on your actual cycle length or an early ultrasound measurement instead.
How is 'weeks pregnant' counted from the last period?
From the first day of the last menstrual period, not from conception — which is why pregnancy is conventionally described as 40 weeks even though conception typically happens around week 2 of that count.
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.