During which period of pregnancy may drug exposure cause meromelia, cleft lip, and enamel hypoplasia?

Meromelia (Malformed limbs) : The critical period is 20-36 days after fertilization or the first trimester when the organs are just beginning to develop. Drugs such as thialides should be avoided.

Cleft Lip with/without Cleft Palate: Usage of drugs such as topiramate and valproic acid, during the first 3 months of gestation increases the risk of having a baby with this defect.

Enamel Hypoplasia (teeth enamel is hard but thin) : The use of tetracyclines can cause this, as well as yellow-to-brownish discoloration of the teeth. It is important to avoid such drugs during the 2nd and 3rd trimesters.