Normal development of early human embryos: Observation of 90 specimens at Carnegie stages 7 to 13

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Abstract
1. A systematic analysis of the relation between 2 parameters of developmental stage — clinically assessed embryonic age and growth — was made for 90 human embryos at Carnegie stages 7–13 from healthy pregnancies. 2. The data showed remarkable individual variation in the relation between age and developmental stage of embryos at stages 11–13. It appears that the mean age for these stages in our specimens was a few days greater than the corresponding age in the currently cited standards, although a definite conclusion must await further studies. 3. The relation between body length and developmental stages in our embryos was not different from the corresponding standards presented by previous investigators. The relation of the number of somites to developmental stage and greatest length in 13 embryos at stages 9–11 was approximately in accord with that reported for several corresponding embryos by other investigators. 4. The oldest age for embryos at each stage in our data may have practical use in denying a causal relation between the exposure of pregnant women to an exogenous agent at such a stage and the subsequent appearance of specific malformations in their progeny.
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