Crown-rump length (CRL) is the single most accurate way to date a pregnancy. Measured on a first-trimester ultrasound and run through the Robinson & Fleming (1975) regression, it estimates gestational age to within about 3-5 days — tighter than dating by last menstrual period. This calculator applies that formula directly to your ultrasound's CRL value and scan date.
How CRL dating works
Early in pregnancy, before individual fetal organs are large enough to measure reliably, the embryo grows at a remarkably consistent rate across pregnancies. Robinson and Fleming exploited this by measuring CRL on 334 pregnancies of known gestational age and fitting a regression curve: GA (days) = 8.052 × √(CRL (mm) × 1.037) + 23.73. Because embryonic growth is so uniform in this window, the resulting estimate has less biological variability — and therefore less dating error — than measurements taken later in pregnancy, when individual growth differences between fetuses widen.
The formula was derived and validated for CRL values between 5mm and 84mm, corresponding to roughly 6 to 14 weeks of gestation. This calculator will still compute a result outside that range, but flags it as outside the validated window — other biometry (biparietal diameter, head circumference, or femur length) becomes the more reliable dating measurement once the embryo grows past CRL's useful range.
CRL dating vs. LMP dating
The traditional alternative is Naegele's rule: adding 280 days to the first day of your last menstrual period (LMP). That method depends on an accurately recalled LMP date and a regular ~28-day cycle — assumptions that frequently don't hold. CRL dating measures the embryo directly, sidestepping both problems, which is why ACOG Committee Opinion No. 700 and equivalent guidance from NICE, RCOG, and ISUOG all recommend ultrasound-based dating over LMP dating whenever a first-trimester scan is available.
If you're looking for LMP-based dating instead, see the site's Due Date Calculator, which implements Naegele's rule directly.
Limits and how the result should be used
This calculator implements the published formula exactly, but a due date, once established by an early, accurate ultrasound, is not meant to be casually reset by every subsequent scan. ACOG guidance recommends dating be fixed by the earliest reliable measurement and only revised later if a subsequent scan's estimate diverges by more than a published threshold for that gestational-age window — ask your obstetric provider before assuming a later scan changes anything.
This tool is for education and general reference only. It does not replace the official interpretation in your ultrasound report or the clinical judgment of your obstetric provider, who has access to your full history and the actual scan images.