Table 1: Approaches used by six previous studies to generate CRL reference charts.

Robinson Robinson/Fleming Hadlock Daya Westerway (ASUM) Verburg Year 1973 1976 1992 1993 2000 2008 Probe used TA TA TA TA, TV TA, TV TA, TV Method to calculate gestation at ultrasound LMP LMP LMP IVF dates LMP LMP n * 214 (80) 334 452 (416) 94 478 3760 Gestational ages that reference ranges were reported (weeks) 6–14 6–14 5.7–18 6.1–13.3 5.2–14.4 6–15

Scan: type of ultrasound scanner used, technique: technique used for scanning, TA: transabdominal, TV: transvaginal, RT: real time, LMP: last menstrual period, IVF: in vitro fertilisation. *n represents the number of observations, number of patients in brackets if this is different to number of observations.