The reproducibility of fetal crown rump length measurements obtained with real time ultrasound systems compared with those of a conventional B-scanner.

Comparison of the reproducibility of fetal crown rump length measurements (CRL) obtained in a resting state by sonar with a phase focused linear array real time scanner, a real time mechanical sector scanner, and a static B-scanner did not show any significant differences between machines. Maternal movement was found to produce a change in fetal intrauterine position and to induce fetal movement. Examination following maternal movement was felt to be more representative of clinical conditions and produced greater variability of CRL measurement, shown by all machines to a differing extent, although the differences between machines were not substantial. There is some evidence that the discernment of fetal movement and the quality of fetal echoes obtained with different machines affects the reproducibility of CRL measurement. Thus variability of CRL measurement with every machine is small in terms of gestational age and justifies the use of real time machines to establish gestational age.