When are teratogons most harmful to fetus?

Teratogens are most harmful to a fetus during the critical periods of organogenesis, which typically occurs between the 3rd and 8th weeks of pregnancy. During this time, the major organs and structures are developing, making the fetus especially vulnerable to environmental influences. Exposure to teratogens during this period can lead to congenital anomalies and developmental issues. However, some teratogens can also have effects later in pregnancy, depending on the timing and nature of the exposure.