Cytomegalovirus in Pregnancy: How to Interpret IgM, IgG and Avidity

A positive cytomegalovirus (CMV) test during pregnancy often sounds more alarming than it is. A single IgG or IgM result cannot tell you all at once when infection occurred, whether the fetus is infected, or whether the baby will be affected. Clinicians assess several pieces together: previous results, IgG, IgM, IgG avidity, gestational age, and ultrasound findings.

CMV belongs to the herpesvirus family. After infection, it remains latent in the body and can occasionally reactivate. Most healthy adults have no symptoms or develop a mild mononucleosis-like illness. During pregnancy, the key question is not simply whether someone encountered CMV in the past, but whether a primary infection may have occurred recently and whether the virus reached the fetus.

A positive IgG result does not mean active infection

IgG represents longer-term immune memory. According to the CDC, a positive CMV IgG result in a person older than 12 months means infection occurred at some point in life, but it does not show when. By itself, it does not prove reactivation, explain symptoms, or diagnose fetal infection.

IgG also does not provide absolute protection: fetal transmission can follow a primary infection, reinfection with another CMV strain, or reactivation. However, the risk of transmission after a non-primary infection is much lower than after a first infection during pregnancy.

How to interpret IgM, IgG and avidity

IgG negative, IgM negative

There is no serologic evidence of previous infection. This is not an illness, but it indicates susceptibility to primary infection. Whether repeat testing is appropriate depends on local guidance, gestational age, and the clinical situation.

IgG positive, IgM negative

This most often reflects infection in the past. If the sample was obtained early in pregnancy and there are no suspicious symptoms or ultrasound findings, a recent primary infection is less likely. Still, no result should be interpreted without clinical context.

IgM positive

IgM alone cannot diagnose a primary CMV infection. It may persist after an older infection, appear during a non-primary infection, or be falsely positive. A positive IgM result therefore needs assessment with IgG and, usually, an IgG avidity test.

What IgG avidity shows

Avidity describes how strongly IgG antibodies bind to the virus. It is usually low after a primary infection and rises over roughly 2–4 months. Positive IgM together with low avidity supports a recent primary infection. High avidity in the first trimester makes a primary infection immediately before or early in pregnancy less likely. An intermediate result remains uncertain.

The cutoffs for low, intermediate, and high avidity vary between laboratory systems. Later in pregnancy, high avidity may not reliably date infection. The sampling date, previous results, assay used, and specialist interpretation all matter.

What to do after a concerning result

1. Contact your obstetric clinician or a maternal-fetal medicine specialist promptly, especially after positive IgM, low avidity, or seroconversion from negative to positive IgG.
2. Gather the exact dates of pregnancy, symptoms, and every test. If a pre-pregnancy or early-pregnancy blood sample is stored, comparing samples may help date infection.
3. Do not try to prove “activity” with repeated blood or urine PCR tests without a specialist plan. The European ECCI consensus does not recommend these tests for dating primary infection in women with positive IgG and IgM.
4. Do not start antivirals on your own. Management depends on the likely infection date, gestational age, local protocols, and potential treatment risks.

Maternal blood tests do not show whether the fetus is infected

Even a confirmed primary infection in pregnancy does not automatically mean that the fetus is infected or harmed. PCR testing of amniotic fluid is the most informative prenatal test for fetal CMV. SMFM advises amniocentesis after 21 weeks and at least 6 weeks after likely maternal infection. Timing differs somewhat across protocols, so a maternal-fetal medicine specialist should determine it.

Ultrasound is used to look for possible effects and follow fetal development, but a normal scan does not rule out infection. Confirmed maternal or fetal infection may lead to serial expert ultrasound and, in selected cases, fetal MRI.

Why treatment decisions require a specialist

Guidelines differ. The updated RCOG paper reports reduced transmission with early valaciclovir after confirmed primary infection in the first 12 weeks. In contrast, SMFM limits antenatal antiviral therapy to research protocols. This reflects different assessments of emerging evidence and different national practice, not a treatment choice to make without medical supervision.

Any decision requires a confirmed diagnosis, an estimate of infection timing, review of kidney function and drug interactions, and monitoring. A positive IgG result by itself is not an indication for treatment.

Reducing the risk of infection

Young children can shed large amounts of CMV in saliva and urine. People who live or work with young children can reduce exposure by:

• washing hands with soap after diaper changes, helping with toileting, or contact with saliva;
• not sharing spoons, cups, food, or toothbrushes with a young child;
• avoiding kisses on a young child’s lips; kissing the forehead or hugging is safer;
• cleaning surfaces and toys contaminated with saliva or urine.

These measures reduce contact with body fluids but cannot guarantee complete protection. They should not prevent normal caregiving or affectionate contact with a child.

What matters after birth

If CMV infection was suspected or confirmed during pregnancy, discuss newborn testing before delivery. Congenital CMV is confirmed by saliva PCR with urine confirmation, collected within the first 2–3 weeks of life. Later testing cannot reliably distinguish infection acquired before birth from infection acquired after birth. Newborn IgG or IgM antibody tests are not suitable for this confirmation.

Most infants with congenital CMV have no signs at birth, but hearing loss can appear later in some children. Timely diagnosis allows appropriate hearing, vision, and developmental follow-up. If you receive a positive IgM or conflicting results, the most useful next step is not random repeat testing but a prompt specialist plan.

Sources

• CDC: Clinical Overview of CMV and Congenital CMV
• CDC: Laboratory Testing for CMV and Congenital CMV
• ECCI: European consensus recommendations, 2024
• RCOG: Congenital CMV update
• SMFM Consult Series #39

Author: АС