A worked example
A last period starting April 1, 2026 gives an estimated due date of January 6, 2027 — exactly 280 days later.
Frequently asked questions
How accurate is an LMP-based due date?
It assumes a textbook 28-day cycle with ovulation on day 14 — for people with longer or shorter cycles, the actual due date can shift by several days either direction. An early ultrasound dating is generally more precise.
Why 280 days specifically?
That's 40 weeks, the traditional full-term length counted from the last period (not from conception, which happens roughly two weeks later) — a convention called Naegele's rule.
Will my baby actually arrive on this date?
Unlikely as the exact day — only a small percentage of babies are born precisely on their estimated due date. It's a useful planning target, not a firm deadline.
This calculator provides estimates for general informational purposes only and is not medical advice.