Teratogenesis is most likely in ______ fetal age.

Correct option is B

Sol. Teratogenesis is most likely to occur between 20 and 56 days after fertilization, a period known as the embryonic stage. This is the critical period for organogenesis, during which major organs and body structures are forming. Exposure to teratogens such as drugs, radiation, infections, or chemicals during this time can cause congenital malformations.
Explanation of each option:
· (a) Before 20th day after fertilization – Incorrect. The first 20 days mainly involve cell division and implantation, and while damage at this stage can lead to pregnancy loss, major congenital malformations are unlikely.
· (b) Between 20 and 56 days after fertilization – Correct answer. This period, also called the organogenesis phase, is when the developing fetus is most susceptible to teratogens, leading to birth defects if exposed to harmful agents.
· (c) In second trimester – Incorrect. By the second trimester, most major organs are already formed, and the fetus is in a growth phase. While some exposures can still be harmful, the risk of major structural malformations is lower.
· (d) In third trimester – Incorrect. The third trimester focuses on fetal growth and maturation, and while teratogens can still impact development (e.g., affecting brain growth), the risk of structural malformations is significantly reduced.