Updating live using Naegele's rule — LMP + 280 days (40 weeks), adjusted for your cycle length.
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What your result means
The estimate uses Naegele's rule (LMP + 280 days), the standard adopted by ACOG and the WHO. Only about 4% of babies arrive on the exact date — most come within two weeks on either side.
Weeks 1–12 are the first trimester, 13–26 the second, 27–40 the third. Your current week sets the context for screening tests, symptoms, and appointments.
A first-trimester dating ultrasound is more accurate than LMP math, especially if your cycle isn't 28 days. If the ultrasound differs by more than about a week, clinicians use the ultrasound date instead.
"Full term" is 39–40 weeks; 37–38 is "early term," and 41+ is "late term." Your provider decides when induction gets discussed, usually at or after 41 weeks.
Quick questions
Why is my due date different at the OB?
First-trimester ultrasound dating (especially between 7 and 13 weeks) is more precise than LMP math. Your clinician will override the LMP estimate when the ultrasound differs by more than 5 to 7 days.
Can I calculate from an IVF transfer date?
Yes. For a 5-day embryo transfer, the due date is 261 days after transfer. For a 3-day transfer, add 263 days. Use the conception date field with the corresponding offset, or confirm the exact date with your clinic.
What if my cycle isn't 28 days?
Naegele's rule assumes a 28-day cycle. Longer cycles push your due date later; shorter cycles pull it earlier. The cycle-length field above applies that correction automatically, but a first-trimester ultrasound is still the gold standard.
Informational only and not medical advice. Due dates are estimates — only about 4% of babies are born on their calculated date. Talk to your clinician and confirm your date with a first-trimester ultrasound.