The Importance of First Trimester Screening for Maternal CMV Infection

If a woman is infected with cytomegalovirus (CMV) for the first time from about two months before pregnancy up to the first three months of pregnancy, there is roughly a 30% chance that the virus will be passed to the fetus. If the fetus becomes infected, there is a significant risk of serious health problems, such as permanent hearing loss or delays in brain development.

The most common way for adults to catch CMV is through close contact with young children who are already infected. This is why mothers with at least one young child at home are at higher risk.

Risk Timing and Severity

• The risk of passing CMV to the baby is highest if the mother is infected for the first time during the early stages of pregnancy (the first trimester).

• Infection later in pregnancy (after the first three months) is much less likely to cause severe problems for the baby.

Symptoms and Screening

• Most CMV infections in pregnant women cause no symptoms or only mild illness, so relying on symptoms alone is not effective for detecting infection.

• Because of this, routine blood tests to check for CMV antibodies are recommended, especially in the first trimester. These tests look for two types of antibodies: IgM (shows recent infection) and IgG (shows past infection).

Screening Strategy

• If a woman has already tested positive for CMV IgG antibodies before pregnancy, she does not need further CMV screening during this pregnancy. While reinfection or reactivation can occur, the risk of serious fetal complications is very low, and there is no proven treatment in these cases.

• If there is no record of a previous positive CMV IgG test, it is recommended to test for both CMV IgM and IgG at around 12 weeks of pregnancy.

• In our clinic, we do a more cost-effective strategy of doing CMV IgM only at 12 weeks of pregnancy.

  • If CMV IgM is positive, CMV IgG is then done. If CMV IgG is also positive, then CMV IgG avidity test would be tested.

  • If the IgG avidity is low, this suggests a recent infection (within the last three months), and treatment may be started.

  • If the IgG avidity is high, the infection is older (more than four months ago), and the risk to the fetus is very low, so no treatment is needed.

Treatment

• High-dose valaciclovir (2 grams, four times a day) has been shown to reduce the risk of passing CMV from mother to fetus if started soon after infection. The effectiveness of treatment is higher the sooner it is started after infection is detected.

Further Testing

• From 17 weeks of pregnancy onwards, amniocentesis (testing the fluid around the baby) can be performed to check if the fetus is infected with CMV.

  • If CMV is not found in the amniotic fluid, it is very unlikely the baby will have serious problems, and treatment can be stopped.

  • If CMV is detected, the fetus is infected. Continuing valaciclovir may help reduce the risk of severe damage, and close monitoring with ultrasound and possibly fetal MRI can help assess the baby's brain development.

Key Points for Prevention

• Pregnant women, especially those with young children at home, should practice good hygiene, such as frequent handwashing, to reduce the risk of CMV infection.

• Universal screening in the first trimester is recommended in areas where CMV is common or if the mother’s CMV status is unknown.

In summary, early detection and treatment of primary CMV infection in pregnancy can significantly reduce the risk to the fetus. Routine screening, especially in the first trimester, is the most effective way to identify women at risk, as most infections do not cause noticeable symptoms.