One of the functions of the placenta is to act as a filter — allowing beneficial substances such as oxygen and nutrients to pass from the maternal circulation into the fetal circulation, but also trying to prevent harmful substances from making this journey. For example, the placenta allows the passage of some proteins and other bigger chemicals in the blood, including maternal antibodies — chemicals made by the mother to attack any infectious agents, such as bacteria that get into her body while she is pregnant. Allowing her antibodies to pass across the placenta and into the fetus is important in providing protection for the fetus, and later the newborn baby, against the same infectious agents.
However, the placenta cannot prevent the transfer of alcohol, other drugs and viruses to the fetus. These agents can cause birth defects, like discolouration of teeth, bone deformity and brain damage. Advising a pregnant woman to stop taking harmful drugs, and to reduce her exposure to harmful chemicals as much as possible, is very important. Your will learn more about this in Study Session 14 on health promotion issues in antenatal care.