The embryo is most vulnerable to teratogens during which stage of pregnancy?

Organ formation: Starts at 3-8 weeks after conception. Each organ has a a critical period;e.g., heart 3-4 weeks, brain 3 weeks-delivery. In general, the first trimester is the most vulnerable time. Exposure to alcohol, tobacco, drugs ; medications, low folate (folic acid) levels, maternal diabetes, infections, chemicals, ; hyperthermia are some teratogens that can cause harm before a woman knows she's pregnant.