The correct answer is: about 3 weeks after fertilization.
Teratogens are agents, biological, chemical or physical, that can affect normal development causing the formation of anomalies. The critical time for the teratogen’s effects is embryonic development. If teratogens act within two weeks after fertilization (implementation phase), probably the embryo will be lost. Three to eight weeks after fertilization is the embryonic period when primary tissue differentiation occurs and under the influence of teratogen major malformation might occur. During the fetal period (after eight weeks) central nervous system is at risk.