The accuracy of conceptual age estimated by crown-rump length is not fully assessed by Robinson and Fleming in their excellent study.1 They avoid the problem by using the expression “menstrual maturity.” We have evaluated predictors of fetal age from crown-rump length measurements where conceptual age was calculated either from the day of artificial insemination by donor (AID) or from the day of ovulation as judged from recordings of basal body temperature (BBT). When the date of AID was used as the conception date, better mathematical fits (R2 = 0.92) were obtained than when the thermal nadir was used (R2 = 0.86). The distribution around the regression line in the AID group is skewed with the 5th-95th percentile range being 3.7 days above the line and 7.3 days below the line. Similar results were obtained in the BBT group. Possible reasons for these findings are discussed. Indexing Words: Crown-rump length · Conceptual age · Artificial insemination · Maternal basal body temperature.