Gestational age from CRL.

Gestational Age from CRL Calculator: how it works

Hadlock's 1992 equation converts a crown-rump length in centimetres to gestational age in days: 7 x exp(1.684969 + 0.315646c - 0.049306c^2 + 0.004057c^3 - 0.000120456c^4), where c is the CRL in cm. A 20 mm CRL gives 60 days, which is 8 weeks 4 days; Robinson's formula gives the same. This calculator shows both, a due date from the scan date, and the ACOG redating check against your last-period dating.

Two published equations are used. Hadlock and colleagues (Radiology, 1992) built a table from 416 patients with good menstrual dates, covering CRLs from 2 mm to 12 cm at 5 to 18 weeks, with variability of about 8% (2 standard deviations). Robinson and Fleming (1975) fitted gestational age in days as 8.052 x the square root of the CRL in mm + 23.73, for 6 to 14 weeks, accurate to within 4.7 days with 95% probability from one measurement. The exact forms of both equations are taken from the table of published formulas in Zhang and colleagues (BMC Pregnancy and Childbirth, 2025). Gestational age here means menstrual age: weeks from the first day of the last period, and the due date is 280 days from that point.

ACOG Committee Opinion 700 says CRL dating is accurate to within 5 to 7 days up to 13 6/7 weeks, is more accurate earlier, and that the due date should change when the ultrasound differs from last-period dating by more than 5 days before 9 0/7 weeks, or by more than 7 days from 9 0/7 to 13 6/7 weeks. After about 14 0/7 weeks CRL loses accuracy and other measurements are used. The calculator applies those thresholds if you enter your last-period gestational age. Your clinician makes the real decision, and a scan measurement depends on the operator and the machine.

Worked example. CRL 20 mm. Hadlock with c = 2.0: exp(1.684969 + 0.631292 - 0.197224 + 0.032456 - 0.001927) = exp(2.149566) = 8.581, times 7 = 60.1 days, which is 8 weeks 4 days. Robinson: 8.052 x sqrt(20) + 23.73 = 59.7 days, which rounds to 60 days, also 8 weeks 4 days. With a scan on 1 October 2026 the due date is 280 - 60 = 220 days later, 9 May 2027. If last-period dating gave 8 weeks 0 days (56 days) the scan is 4 days ahead, within the 5 day threshold before 9 weeks, so the due date would stay on last-period dating.