Final answer:
Pregnant women are advised to avoid raw fish due to increased risks of mercury poisoning, which can harm fetal development, and due to the heightened risk of Listeria infection, which can cross the placenta and harm the fetus.
Step-by-step explanation:
Pregnant women are advised against eating raw fish primarily due to the risks of mercury poisoning and foodborne illnesses. The process of biomagnification in ecosystems results in higher concentrations of heavy metals like mercury in large predatory fish such as swordfish, shark, king mackerel, or tilefish. The United States Environmental Protection Agency (EPA) therefore recommends that pregnant women avoid these types of fish and opt for options with lower mercury content, such as salmon, tilapia, shrimp, pollock, and catfish.
Moreover, the risk of Listeria infection is another reason for this dietary restriction. Listeria, a pathogen that can result in miscarriage or neonatal infection, can be found in raw or undercooked seafood, as well as in other food items like soft cheeses and refrigerated cold cuts.
Additionally, cooking or freezing fish can help to kill worms and larvae present in the meat, reducing the risk of parasitic infections such as anisakiasis. Proper handling and preparation of fish are critical to avoiding these health risks.