The crown-rump lengths (CRL) of 224 pregnancies which resulted from gamete intra-Fallopian transfer (GIFT) or in-vitro fertilization (IVF) were assessed two to four times in the first trimester. The results were compared with some of the published articles which assessed CRL in spontaneous and induced pregnancies. The range of CRL measurements from this study was generally smaller than the previously published data, although all women went on to deliver normal fetuses at full term weighing > 2.5 kg. It was not possible to identify a clear reason for this finding, but factors which might have been relevant included population differences, more accurate estimation of ovulation/conception time, the exclusive use of vaginal ultrasonography in this study and variations in the embryonic implantation/development times. There is a need to review the commonly used CRL charts in view of the increasing use of transvaginal ultrasonography and the increasing number of pregnancies with known ovulation/conception times. Each centre should aim to establish the normal CRL range for its own population.