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Why can some viruses and drugs affect a fetus during pregnancy?

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Close-up of a home pregnancy test device showing a blue plus sign in the result window, indicating a positive result for pregnancy. — pregnancy Devices for individual use at home detect pregnancy by measuring levels of the hormone human chorionic gonadotropin (hCG) in urine.

Viruses and drugs can affect a fetus during pregnancy because they are often capable of crossing the placental barrier.123

The placenta serves as the vital organ for nutrient, oxygen, and waste exchange between the mother and the fetus. While this barrier is designed to facilitate these essential exchanges, it is not impenetrable. Many substances, including various viruses and drugs, can filter across this barrier and enter the fetal circulation.124

Factors influencing this transfer include:
* Chemical Properties: Drugs that are fat-soluble are more likely to cross the placental barrier, whereas water-soluble drugs or those with high molecular weights are often impeded.3
* Binding: Drugs that bind to large molecules, such as blood-borne proteins, are less likely to reach the fetus.3

Once these agents cross into the fetal environment, they can act as teratogens—substances capable of causing structural or functional abnormalities, particularly during the first trimester when the embryo is highly susceptible to developmental disruption. Consequently, medical professionals emphasize that any drug therapy during pregnancy must be carefully considered to avoid potential harm to the developing fetus.531