During which stage of pregnancy is the risk of cytomegalovirus transmission to the fetus highest?

Cytomegalovirus (CMV) transmission to the fetus is highest during the third trimester of pregnancy. This increased risk can be attributed to several factors related to both viral replication and fetal development. During the third trimester, the fetus is more developed and has increased vascularization, providing a more robust environment for the virus to cross the placental barrier. Additionally, the maternal immune system undergoes changes during pregnancy that can influence the likelihood of viral transmission.

While CMV can be transmitted during any stage of pregnancy, the rates of congenital CMV infection are most significant in the third trimester, where exposure to maternal shedding of the virus is at its peak. This timing correlates with increased risk factors such as the woman's reactivation of a previously dormant infection or higher viral loads.

In contrast, the first trimester, although a critical period for many infections due to early fetal development, does not present the same heightened transmission risk for CMV as the later stages of pregnancy. The second trimester also exhibits lower transmission rates when compared to the third. Understanding these dynamics is vital for prenatal care and infection prevention strategies.