Ultrasound evidence for variability in the size and development of normal human embryos before the tenth post-insemination week after assisted reproductive technologies

Differences in human chorionic gonadotrophin (HCG), averagechorionic sac diameter, embryo/fetus crown —rump lengthand biparietal diameter were determined in 107 singleton pregnanciesdelivered after 34 weeks, whose post-insemination age was preciselyknown as a result of in-vitro fertilization (n = 28) or gameteintra-Fallopian transfer (n = 79). Crown — rump lengthswere interpreted in relation to the developmental stage of theembryo. A 7-fold to 10-fold difference in HCG levels was observedon post-insemination days 13–16. A 2-fold to 3-fold differenceoccurred in average chorionic sac diameter on days 25–36.A 2-fold difference occurred in crown — rump length onseven of 10 days prior to day 44. Biparietal diameter differedby no more than 42% from day 57–68 and 20% after day 68.Chorionic sac diameter, crown -rump length, and biparietal diameterwere also related to newborn weight. The results suggest firstlythat marked differences occur in the rate of early human development,secondly, that the differences occur prior to day 27 when observablecardiac activity begins, and thirdly, that the differences areminimized after day 68 when the embryonic period of developmentis completed.