During which trimester should sulfonamides be avoided in pregnant patients due to the risk of hyperbilirubinemia in the newborn?

Sulfonamides should be avoided during the third trimester of pregnancy primarily due to the risk of hyperbilirubinemia in the newborn. This risk is associated with the ability of sulfonamides to displace bilirubin from its binding sites on albumin. Increased levels of free bilirubin can lead to jaundice and potentially cause kernicterus, a serious condition where bilirubin accumulates in the brain, causing neurological damage.

During the third trimester, the fetus is at a greater risk as the liver is becoming more mature but may still not be fully capable of handling increased levels of bilirubin efficiently. Additionally, sulfonamides can cross the placenta, which further exposes the fetus to the potential adverse effects.

The concern is less pronounced in the first and second trimesters, where the fetal physiology and the risks of sulfonamide exposure are different. In those earlier trimesters, the main concerns revolve around potential teratogenic effects rather than the direct impact on bilirubin metabolism during the later stages of gestation. Hence, the third trimester is the critical period where the use of sulfonamides should be avoided due to these specific risks.