Developmental Psychology critical period: brain is set to acquire a function during a limited period of time e.g. language acquisition has a critical period sensitive period: progression will be impaired, but can be overcome critical periods of vulnerability - e.g. severe mental retardation will occur if a mother contracts measles while gestating in germinal period, 20% of pregnancies spontaneously abort in embryonic period, 33% of pregnancies spontaneously abort vulnerable to outside influence (i.e. birth defects can occur) teratogens: environmental agents which can harm the fetus fetal alcohol syndrome does not always occur malnutrition can have a strong effect on birth defects smoking is linked to miscarriages, premature births, stillbirths, sudden infant death syndrome