What are the critical periods in prenatal development?

Critical periods are windows during prenatal development when specific organs and systems are most vulnerable to disruption from teratogens, which are harmful agents like alcohol, drugs, infections, and radiation. The embryonic period from weeks three through eight is the most critical because major organs and body structures are forming. Alcohol exposure during this time can cause fetal alcohol spectrum disorders. Rubella infection in the first trimester can cause heart defects, deafness, and intellectual disability.