Several factors influence whether a substance can cross the placental barrier, including its molecular weight, lipid solubility, and degree of ionization. For instance: - Alcohol and nicotine can easily cross the placental barrier, contributing to fetal alcohol spectrum disorders and low birth weight, respectively. - Certain medications, such as thalidomide and isotretinoin, are notorious for their teratogenic effects. - Heavy metals like lead and mercury can also cross the barrier and impact fetal development.
