CMV infection can pass from mother to baby during pregnancy. Here’s how to prevent it

Pregnant women are routinely advised to avoid certain soft cheeses and take care when handling cat litter due to the risk of harmful infections.

Far fewer are told not to share food, drinks or utensils with young children, and to wash their hands after changing nappies or wiping noses. Yet these simple precautions can reduce infection from a virus called cytomegalovirus, or CMV.

CMV is the most common infection passed from mother to baby during pregnancy and is a preventable cause of childhood disability.

Early testing and targeted education for CMV-negative women has been shown to reduce infection rates from 7.6% to 1.6%. This means only 17 at-risk pregnant women need to be given this information to prevent one CMV infection during pregnancy.

How does it cause illness?

CMV spreads through direct contact with body fluids, particularly the saliva, nasal mucus and urine of young children.

CMV can cause a mild flu-like illness, with a fever and swollen lymph nodes, or no symptoms at all.

Young children are a common source of exposure because they can shed high levels of the virus in their body fluids for long periods while remaining well. Parents, carers and childcare workers are therefore at increased risk of infection.

But when a pregnant person acquires CMV, in some cases the virus can cross the placenta and infect the developing fetus. Around one in 200 babies is born with congenital CMV, meaning the infection has passed from mother to baby during pregnancy.

While most babies will be well, the infection and resulting inflammation can interfere with fetal development. In some babies this can result in microcephaly (where the baby’s head is smaller than expected), hearing loss, developmental delay and cerebral palsy.

How is a mother’s CMV infection diagnosed and treated?

A blood test is the main way to detect an infection in a pregnant woman.

Until recently, routine CMV blood testing in pregnancy was not recommended because there was no proven treatment to reduce transmission from the mother to the developing baby in the womb.

But evidence now shows that if a woman has a primary (first-time) infection in early pregnancy, prompt treatment with the antiviral drug valaciclovir can reduce the risk of CMV passing to the fetus.

This 2020 randomised trial found that among women who had a primary infection in the first trimester, CMV transmission to the fetus occurred in 48% of those given placebo, compared with only 11% of those treated with valaciclovir.

Valaciclovir is now being offered to women with first trimester CMV infection to reduce the risk of fetal infection. Treatment should ideally begin within six weeks of infection and by 14 weeks of pregnancy.

Specialist oversight and regular blood tests are because of the potential side effects of valaciclovir for the mother, including kidney impairment, low white cell count, and anemia.

In Australia, treating a pregnant woman with high-dose valaciclovir to prevent fetal infection is currently available off-label, meaning it hasn’t been formally approved for this specific use in Australia. This is because further research is needed on the benefits and risks of this treatment, including data on the long-term outcomes for mothers and babies.

Australia has strengthened its advice on targeted testing

Australian guidelines now recommend early CMV testing for women at increased risk of exposure. This includes women who work with young children or care for young children of their own.

Testing can identify women who have never previously had CMV. These women remain susceptible to a primary CMV infection during pregnancy, which carries the greatest risk to the baby.

Australian guidelines now recommend all pregnant women be given information about CMV, the potential risk to their baby, and simple hygiene precautions to reduce their risk of infection.

Would screening everyone be fairer?

Targeted testing of pregnant women with frequent close contact with young children may apply to close to half of pregnant Australians. But targeted testing depends on women and clinicians knowing CMV exists, recognising who is at increased risk, and remembering to test.

In 2024, a group of European expert clinicians recommended testing all women for CMV. Several countries have now begun to implement these models of care.

But Australian guidelines don’t yet recommend universal screening. More research is needed to determine if this would be cost-effective and feasible, including whether Australia’s health system has the workforce and capacity to implement it safely and equitably.

In the meantime, every pregnant woman should receive information on simple hygiene precautions that can reduce the risk of CMV infection, particularly in the first trimester:

• washing hands after changing nappies or wiping young children’s noses
• kissing young children on the forehead, rather than the mouth
• avoiding sharing food, drinks or utensils with young children.