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

RobinsonRobinson/FlemingHadlockDayaWesterway (ASUM)Verburg

Year197319761992199320002008
Probe usedTATATATA, TVTA, TVTA, TV
Method to calculate gestation at ultrasoundLMPLMPLMPIVF datesLMPLMP
n *214 (80)334452 (416)944783760
Gestational ages that reference ranges were reported (weeks)6–146–145.7–186.1–13.35.2–14.46–15

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.