Human Embryogenesis, Fertilization And Fetus Development

Toxic exposures during the first two weeks following fertilization (second and third weeks of gestational age) may cause prenatal death but do not cause developmental defects. Instead, the body performs a miscarriage. On the other hand, subsequent toxic exposures in the embryonic period often cause major congenital malformations, since the precursors of the major organ systems are developing.
Fetus
A fetus (/ˈfiːtəs/; also spelled foetus, fœtus, faetus, or fætus) is a developing mammal or other viviparous vertebrate after the embryonic stage and before birth.