Development and Intrauterine Fate of Normal and Abnormal Human Conceptuses

Abstract

Using the data from the Kyoto Collection of Human Embryos, three main topics related to normal and abnormal development of human embryos are discussed. 1) Wide variability was noted in developmental stage of human embryos at any given gestational age. This was true not only for the estimated ovulation age but also for ‘coital’ age in single coital cases. Such diversity in human prenatal development may be, at least in part, ‘normal’ biological variability and it should be taken into account when assessing the teratogenic risk of environmental agents to human embryos. 2) At the early postimplantation period prior to major organogenesis, the percentage of morphologically abnormal embryos is high (> 30%), which supports the clinical finding that a substantially large proportion of human conceptuses are eliminated at an early stage of pregnancy, often without the knowledge of the mother. The fate of undifferentiated abnormal embryos is not certain and should be studied. 3) Life‐table estimates for normal and abnormal human conceptuses showed that more than 10% of all embryos recognizable at 5 weeks gestation are malformed or ‘potentially’ malformed. Because of selective intrauterine death of malformed embryos and fetuses, the proportion of the malformed drops to 2.4% by age 8 weeks and 1% at term. The cumulative intrauterine mortality rate of malformed conceptuses was estimated to be 93%, while the corresponding rate for normal conceptuses was 18%. Copyright © 1991, Wiley Blackwell. All rights reserved