Specified critical period of different congenital abnormalities: a new approach for human teratological studies.

The first trimester has been evaluated at different exposures as the critical period of congenital abnormalities. However, the first trimester concept has no scientific basis because environmental agents (teratogens) cannot induce congenital abnormalities during the first gestational month calculated from the first day of the last menstrual period. In addition, the critical period of some congenital abnormalities, for example, hypospadias occurs after the first trimester). Different congenital abnormalities have different critical periods and computer-based evaluation programs help to evaluate different specified congenital abnormalities according to their specific critical periods separately. Thus, this new and feasible approach is recommended for epidemiological and clinical studies in the future after an international consensus in the specified critical periods of different congenital abnormalities. The author of this Commentary proposes a detailed database for this approach for discussion because a consensus may be achieved only after the confirmation or modification of these critical periods.