Which infection is more likely to affect a fetus when the mother is infected after the first trimester?

When considering infections that can affect a fetus if the mother is infected after the first trimester, syphilis stands out. While all the infections listed can have adverse effects on a fetus, the timing of maternal infection relative to fetal development plays a significant role in the severity and likelihood of transmission.

Syphilis, caused by Treponema pallidum, can be transmitted transplacentally at any time during pregnancy, but the risk of severe fetal complications, such as congenital syphilis, increases with maternal infection in the latter stages of pregnancy. If a mother is infected during the second or third trimester, the fetus has a higher chance of being affected compared to infections that predominantly affect the fetus in early pregnancy.

In contrast, infections such as HIV, rubella, and toxoplasmosis have their highest risks of transmission and resultant fetal damage when maternal infection occurs during the first trimester. For example, rubella poses the greatest risk of congenital defects if the mother is infected early in pregnancy. Toxoplasmosis also has significant risks in early pregnancy, although it can still affect a fetus later on, the risks of severe consequences are notably lower.

Therefore, among the infections listed, syphilis is the most concerning when the mother contracts it after