The goal of this study was to determine the accuracy of an every-day practice for assessing gestational age by ultrasound measurement of the greatest embryonic length (GEL). This retrospective study used measurements taken during the first trimester. We considered all births in this hospital between 1 January 1992 and 31 December 1994 from pregnancies that began by an in-vitro fertilization procedure (IVF). We examined 143 consecutive files, containing 257 measurements made by 72 different operators. The precision of seven embryo growth curves was compared. We calculated for each curve its ability to predict (95% prediction interval) the date the pregnancy began, using these dated pregnancies. For GEL measurements between 3 and 80 mm, which includes most of our population, Robinson and Wisser (2) were the most appropriate curves. The 95% prediction interval was 9.5 and 10.2 days respectively. Dating pregnancies in every-day practice with GEL is nearly as accurate as prospective studies with only one or two scanners.