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Enter the first day of your last menstrual period and get the estimated due date by Naegele's rule — 280 days from that date, adjusted for your average cycle length — plus where you are right now: gestational week, trimester and days to go. It's the same first-pass estimate clinicians use before a dating ultrasound refines it.
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Frequently asked questions
How accurate is a due date from the last period?
It's an estimate with a built-in spread: only about 1 in 20 babies arrive on the exact date, and full-term spans 37–42 weeks. A first-trimester ultrasound dates the pregnancy more precisely and will override the calendar estimate.
Why does cycle length change the date?
Naegele's rule assumes ovulation on day 14 of a 28-day cycle. Longer cycles usually mean later ovulation, shifting conception — and the due date — by the difference. That's the adjustment this calculator applies.
What counts as the first day of the last period?
The first day of actual flow — not spotting, and not the day it ended. If your cycles are irregular enough that the date feels arbitrary, say so at your first appointment; dating by ultrasound matters more in that case.