Practice question

Maximum susceptibility window corresponds to active organogenesis when organ anlagen originate and differentiate. Before week three, totipotency allows compensation or embryonic loss without specific defects. After week eight, organ templates established and growth predominates. Between weeks three and eight neural tube closes, pharyngeal arches form face, limb buds outgrow, heart septates, all highly dependent on morphogen gradients. Interruption causes severe structural anomalies like spina bifida, cleft lip, phocomelia. Hence greatest risk for major anatomical birth defects occurs during weeks three to eight, defining critical period for most classic teratogens affecting structural development.