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Which substances can cross the placental barrier?
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Devices for individual use at home detect pregnancy by measuring levels of the hormone human chorionic gonadotropin (hCG) in urine.
The placental barrier serves as the interface between maternal and fetal bloodstreams. While it is often described as the leakiest of the body's distribution barriers, it is not a complete wall; rather, it acts as a selective filter that allows many substances to pass from the mother to the fetus.12
Substances that can cross the placental barrier include:
- Nutrients and Gases: Essential components such as nutrients, water, salt, and oxygen diffuse across to support fetal development, while waste products like carbon dioxide diffuse out.13
- Antibodies: Protective proteins formed by the mother can cross the barrier to provide passive immunity to the fetus.3
- Drugs and Chemicals: Most drugs administered to the mother can pass through the placenta. This includes anesthetics, analgesics, sedatives, antibiotics, and antihypertensive drugs. Lipid-soluble (fat-soluble) molecules cross the barrier particularly readily.425
- Infectious Agents: Various viruses, bacteria, spirochetes, and protozoa can be transmitted from the mother to the fetus through the placenta.4
- Hormones: Many hormones are capable of filtering across the barrier.1
- Alcohol and Narcotics: Substances such as alcohol, heroin, and morphine can cross, potentially leading to severe developmental consequences for the fetus.46
Conversely, the transfer of large-molecular-weight molecules is generally limited, and the barrier typically prevents the passage of maternal blood cells and bacteria under normal conditions.12