The critical point in a developing fetus falls roughly within the 5th week and the 6th month. This period is defined as the critical period because beginning around the 5th week tissue begins to differentiate. This specialized tissue starts developing from the top down. Specialized tissues begin forming limbs and digits from the top of the fetus and working its way down to its limbs.
Any agent that causes a birth defect is called a teratogen. Teratogens can vary from chemicals to abnormal heat generated by the mother's body. Defects can be anything from missing limbs to cleft lips or palettes. The degree of severity varies according to the amount of time and the amount of teratogens exposed to.
Alcohol is considered one of the most preventable teratogens, but still effects thousands of babies each year. Fetal Alcohol syndrome is a birth defect caused by consumption of alcohol during pregnancy. The difference between Alcohol and other teratogens such as thalidomide is alcohol attacks the developing brain during the entire pregnancy as opposed to the developing limbs during the critical period. During development of the fetus the brain is constantly developing. This leaves it open to attack from alcohol at all times during the developing months. "Fetal Alcohol Disorders can cause mental retardation, facial deformities, stunted physical and emotional development, behavioral problems, memory deficiencies, attention deficits, impulsiveness, an inability to reason from cause to effect; a failure to comprehend the concept of time; and an inability to tell reality from fantasy."
Thalidomide is an interesting teratogen because it works in a different way from alcohol. Thalidomide was a drug prescribed by doctors to help women deal with morning sickness during pregnancy. The unknown side effects of this drug were it inhibited development of the limbs by stopping blood vessel development. Morning sickness usually starts roughly...