The developing fetus

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 (white 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 six weeks corresponds to a menstrual age (gestational age) of eight weeks. Embryonic weeks 1 to 8 are considered the embryonic period of development and are followed by the fetal period of development.