The placenta is a complete barrier that protects the fetus from medicines, chemicals, and infections in the mother's blood.

The placenta connects the developing fetus with the mother’s circulation, but the two blood supplies do not simply mix together.

That led to the useful idea of a placental barrier. The placenta does filter and regulate exchange—but “barrier” can sound much more absolute than the biology really is.

Many substances must cross the placenta for pregnancy to work. Oxygen and nutrients move toward the fetus, while carbon dioxide and other waste products move back toward the mother. Maternal antibodies can cross as well and provide the newborn with temporary immune protection.

Harmful exposures are not automatically excluded.

Doctors learned that some infections can pass across the placenta or damage it. Measurements showed that many medicines and chemicals taken by the mother also reach the fetus.

The thalidomide disaster made the danger especially clear. Thalidomide taken during early pregnancy could reach the developing embryo and cause severe limb and organ malformations.

Modern placental biology therefore treats the organ as a selective interface. Whether a substance crosses depends on factors such as molecular size, fat solubility, transport proteins, metabolism, placental health, and the stage of pregnancy.

Crossing the placenta does not automatically mean causing harm. Dose and timing matter greatly. The correction is that the placenta regulates exposure rather than guaranteeing fetal isolation.