Due Date from Ultrasound Calculator

About this calculator

An early ultrasound measures gestational age directly from fetal size — crown-rump length in the first trimester, or biparietal diameter and femur length later — which is more accurate than dating from last menstrual period alone, especially for people with irregular cycles or uncertain conception dates. From that measured gestational age, the estimated due date is simply 40 weeks from the calculated start of pregnancy, adjusted for how many weeks along the scan showed you to be.

Obstetricians use the ultrasound-based due date as the official pregnancy dating reference once it’s established, and it typically takes priority over an LMP-based estimate when the two disagree by more than about a week, since ultrasound dating in the first trimester carries a much narrower margin of error. Expectant parents use this figure to track weeks remaining and time key milestones like the anatomy scan and third-trimester visits.

This calculator takes the gestational age measured at your ultrasound and the date of that scan, and returns your estimated due date along with the days and weeks remaining from today.