The estimated age of staged human embryos and early fetuses

The appearance of anatomic features during embryogenesis relative to menstrual history is not well established. The 621 staged human embryos and fetuses in the Carnegie Embryological Collection were examined, and morphologic features were recorded in 494 specimens in good condition, from 382 intrauterine, 49 ectopic, and 55 pregnancies of other or unknown source. There were menstrual data on 276 specimens, and for 10 the date of a single coitus was known. The embryo develops to 1 mm at the twenty-eighth menstrual day, after which its crown-rump length increases by 0.7 mm per day. Coital ages and postovulatory ages estimated from menstrual data correlated significantly (p less than 0.001) with crown-rump length and Carnegie stage, but estimated postovulatory age was highly variable (standard deviation, 10.5 days). Morphologic features appeared within a narrow interval (standard deviation, 0.0 to 1.5 Carnegie stages). Normal human embryogenesis is a stereotyped sequence with little statistical variation, but menstrual data in individual cases may be unreliable in dating this sequence.