The genetic factors and gene-environment interactions influence the receptiveness to teratogenic effects. The third principle of teratology describes the mechanism through teratogenic agents that initiate an abnormal embryo-genesis.
Teratogenic effects depend on the stage of development; if teratogenic agents act during the pre-embryonic period, they can cause a spontaneous abortion, while, if teratogenic agents act during the embryonic period, they lead to major birth defects.
The maternal and fetal genotypes can influence the teratogenicity of exposure, hence, different women and children display different adverse effects, though they were exposed to the same amount of agents and doses in the same exposure periods.
The most sensitive developmental period to exposure to teratogenic agents is the embryonic period. The embryonic period begins in the second week after conception and ends in the eighth week after conception. It seems to be the most defenseless period of time for the majority of teratogenic exposures.
Teratogenic effects depend also on the dose of teratogenic agent. If the agent that is considered non-teratogenic surpasses an established threshold, it could lead to maternal toxicity and, therefore, become teratogenic.
Teratogenic effects depend also on the route of exposure. If the route of exposure allows for systemic absorption, then the agent presents the risk to become teratogenic and to reach the embryo, causing adverse effects.