What term is given to the time period during which irreversible damage to the fetus may occur from specific events, such as malnutrition or exposure to toxins?

 Critical period 

Critical period is the time of development in which the body can be very sensitive to damage that may be caused by the administration of drugs, alcohol or other harmful conditions during pregnancy.  At the critical period, all of the baby’s organs start forming and functioning. The developing fetus experiences rapid cell differentiation during this period.