Summary. Fetal crown rump length (CRL) was measured weekly in 33 singleton pregnancies that were established after in vitro fertilization, gamete intrafallopian transfer or natural intercourse in monitored infertility treatment cycles. The time of the luteinizing hormone surge or hCG administration was known for all of them. There was no difference in CRL between the different infertility treatment groups. CRL measurements from this and a similar study where gestational age had been based on the time of ovulation were compared with the CRL measurements from pregnancies where the gestational age had been calculated from the last menstrual period. The CRL was found to be generally smaller in the studies based on ovulation timing and the difference was statistically significant at one time point. This suggests that there is a need for a gold standard in obstetric ultrasound, measurements being based on the gestational age as calculated from the unequivocal date of conception, rather than the last menstrual period.