How this calculator works
The menstrual estimate assumes regular cycles and adjusts the standard 280 days for cycle length. The ultrasound estimate starts from the age documented on the scan. Both estimates are advanced to the same assessment date. The signed difference compares due dates, not fetal growth. A clinician must decide whether redating is warranted based on scan timing, measurement quality and the reliability of menstrual dates; this calculator never automatically selects a new clinical due date. The result is an estimate for information only and is not medical advice.
Formula
LMP EDD = LMP + 280 + cycle length - 28 days. Scan EDD = scan date + 280 - scan age in days. Age on assessment = 280 + assessment date - EDD.
Worked example
LMP 2026-01-01 with a 28-day cycle gives EDD 2026-10-08. A scan on 2026-03-05 reporting 9 weeks gives the same EDD, a difference of zero.
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Frequently asked questions
Why is this separate from a simple due-date calculator?
It compares two dating sources and advances both ages to a common date.
Should I change my due date if they differ?
Discuss it with the maternity team. The size of the discrepancy and gestational age at the scan both matter.
Does a later scan measure a new conception date?
No. Later size differences can reflect growth variation and do not necessarily justify changing the established date.