Teratogens exert maximum effects during:
Teratogenic susceptibility varies with developmental stage. Preimplantation period weeks 1-3 often leads to all-or-none embryonic loss due to totipotency. Organogenesis period spanning weeks 3-8 post-fertilization, when gastrulation, neurulation and formation of heart, limbs, eyes occur, represents peak sensitivity. Exposure to drugs like thalidomide, alcohol, retinoic acid or infections during this window disrupts morphogenetic signaling such as Shh, Wnt, Fgf causing major structural anomalies rather than lethality. After week 8 fetal growth and functional maturation predominate, teratogens tend to cause physiological defects or growth restriction not gross anatomical malformations, making early organogenesis most vulnerable phase.
Ref: Moore & Persaud, The Developing Human, 11th ed., Chapter 8: Teratogen sensitivity peak during organogenesis weeks 3-8.