Alcohol, aspirin, caffeine, cocaine and some viruses like AIDS are known to cross this barrier producing harmful effects in the fetus.
Environmental toxins can also play a role in causing birth defects.
Cigarette smoke can reach the developing fetus and is implicated in low birth weight babies and also in respiratory problems with newborns. If a mother has abused drugs during her pregnancy , the baby may be born addicted to that substance and undergo withdrawal symptoms too. Some of these substances are known to cause developmental delays and learning disabilities later in life. Most problems associated with the placenta have to do with how and where the placenta attaches to the uterine wall. Placenta previa, one of the more common complications occurs when the placenta attaches near the entrance of the cervical canal.