There are critical periods during fetal development when environmental influences can impact the child. The first trimester poses the highest risk of miscarriage as organs are forming. Teratogens are any agents that can cause birth defects, such as drugs, chemicals, and viruses. The thalidomide disaster in the early 1960s showed that a drug prescribed for morning sickness caused limb abnormalities in thousands of babies before being identified as teratogenic. The effects of teratogens depend on the developmental period, target specific tissues, and individual factors like genetics and maternal health.