The specific effect that a teratogen such as thalidomide has depends on when the exposure to the teratogen takes place. This concept is referred to as a "critical period" in development.

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A critical period refers to a specific time during fetal development when a teratogen can cause a particular structural or functional abnormality. The timing of the critical period varies depending on the organ or system that is developing. Therefore, the effect of a teratogen such as thalidomide depends on when it is encountered during embryonic or fetal development.

The Specific Effect That a Teratogen Such as Thalidomide Has