Embryonic development

The black bars represent the critical period during which development may be disrupted by a teratogen resulting in a major structural malformation. Cell differentiation occurs over a longer period (hatched bars); exposure during this period can result in minor structural malformations, growth restriction, or functional deficiency. Note: embryonic age is counted from fertilization, whereas menstrual age (ie, gestational age) is counted from the first day of the last menstrual period. Thus, an embryonic age of 6 weeks corresponds to a menstrual age (gestational age) of 8 weeks.