This survey of the development of a large number of virtually unselected human embryos from healthy women allowed us to establish more reliable standards of normal development with respect to crown-rump length, body weight, and external form than those usually cited. Remarkable variation was noted with regard to the relation between clinical age and these attributes and thus it is concluded that crown-rump length and body weight are more reliable indicators of the general state of development than clinically established age.
Dead embryos occurred with a significantly greater frequency in women with a history of genital bleeding during pregnancy than in those without. Externally malformed embryos were found with increasing frequency with advancing developmental stage, with the maximum figure of 3.92% at horizons 19–23. Malformations observed included external defects such as exencephaly, cyclopia, myeloschisis, cleft lip, and several limb malformations. It is of considerable interest that the incidence of most of these defects was far higher than that observed in newborn infants.