How Crown-Rump Length Dating Works
Crown-rump length (CRL) is the longest straight-line measurement of a fetus from the top of the head to the bottom of the torso, taken on a first-trimester ultrasound. Because embryos grow at a remarkably consistent rate in early pregnancy, CRL is the most accurate measurement available for establishing gestational age (GA) and an estimated due date (EDD). This calculator applies the Robinson and Fleming formula (1975), the equation most widely used in clinical obstetric ultrasound: GA (days) = 8.052 × √CRL(mm) + 23.73.
How the calculation works
Enter the CRL measurement from your ultrasound report and choose its unit (millimeters or centimeters — centimeters are converted to millimeters automatically). The calculator takes the square root of the CRL in millimeters, multiplies it by 8.052, and adds 23.73 to get gestational age in days, which is then converted to weeks and days. If you also enter the date the scan was performed, the tool estimates a due date by adding the remaining days of a standard 280-day (40-week) pregnancy to the scan date: EDD = scan date + (280 − GA in days).
Why CRL is used for dating
Dating a pregnancy from the last menstrual period (LMP) assumes precise recall and a regular cycle, which is often unreliable. First-trimester CRL measurement removes that guesswork and is accurate to within about 5-7 days. ACOG and ISUOG guidance recommend redating a pregnancy to the ultrasound estimate when it disagrees with LMP-based dating by more than roughly 5-7 days in the first trimester, since ultrasound dating at this stage is more reliable than menstrual history.
Limitations and when another method is used
The Robinson formula is validated for CRL values between about 15 mm and 84 mm (roughly 6 weeks 2 days to 13 weeks 6 days). Below that range, the embryo may be too small to measure reliably; above it, the fetus begins to flex and curl, making a single straight-line length less accurate, so sonographers switch to biparietal diameter (BPD) or head circumference for dating in the second trimester. This calculator flags measurements outside the validated range so you know when to rely on another method instead.