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), average chorionic sac diameter, embryo/fetus crown-rump length and biparietal diameter were determined in 107 singleton pregnancies delivered after 34 weeks, whose post-insemination age was precisely known as a result of in-vitro fertilization (n = 28) or gamete intra-Fallopian transfer (n = 79). Crown-rump lengths were interpreted in relation to the developmental stage of the embryo. A 7-fold to 10-fold difference in HCG levels was observed on post-insemination days 13-16. A 2-fold to 3-fold difference occurred in average chorionic sac diameter on days 25-36. A 2-fold difference occurred in crown-rump length on seven of 10 days prior to day 44. Biparietal diameter differed by no more than 42% from day 57-68 and 20% after day 68. Chorionic sac diameter, crown-rump length, and biparietal diameter were also related to newborn weight. The results suggest firstly that marked differences occur in the rate of early human development, secondly, that the differences occur prior to day 27 when observable cardiac activity begins, and thirdly, that the differences are minimized after day 68 when the embryonic period of development is completed.