Cytomegalovirus (CMV)

If you're pregnant, planning a pregnancy, or spending your days around small children, one of the infections that matters most for a baby is a virus most adults already carry without knowing it. A vaginal swab for CMV (cytomegalovirus) shows whether that virus is active in your genital tract right now. When CMV passes to a baby before birth, it's the most common non-genetic cause of hearing loss in children and a leading cause of developmental disability.

Most people with CMV feel nothing, and the virus can stay quiet for years before becoming active again. Blood antibody tests show whether you've ever been infected, and this swab looks for the virus itself at one site on one day. It's a research-grade measurement without agreed cutoffs, and no professional guideline recommends it for screening. Read it alongside your blood work, never on its own.

What a Positive Swab Means

CMV is a herpesvirus. After your first infection it never fully leaves. It settles into a dormant state inside certain white blood cells and can become active again later, and a different strain can infect you even if you've had CMV before.

When the virus is active, it's released into body fluids, including vaginal and cervical fluid. Researchers call this shedding. This test looks for the virus's genetic material in vaginal fluid, so a positive result means CMV was present in your genital tract when you collected the swab. That can happen during a first infection, when an old infection becomes active again, or after infection with a new strain.

How Common Shedding Is

Detection rates vary a lot by population and by how the samples were handled, but shedding is common in women who already carry the virus.

Sources: Tanaka et al. (2006); Sapuan et al. (2026); Hehnly et al. (2022).

What this means for you: if you've had CMV before and your swab comes back positive, you're in common company. Pooled across studies of pregnant women who already carry the virus, about one in five shed it somewhere in the body, and cervical and vaginal fluid had the highest rate of any site. A single positive swab is common and, on its own, doesn't tell you much. The next sections cover what does.

Miscarriage and Preterm Birth

The pregnancy findings for vaginal shedding are thin, and they point in different directions.

In one Japanese study, women with CMV in vaginal fluid early in pregnancy had about seven times the miscarriage rate of women without it. The plausible range was wide, roughly 2 to 24 times, which is what you'd expect when a single cohort has only a handful of miscarriages. This finding hasn't been replicated, and it doesn't show that shedding causes miscarriage. Preterm birth looks different: among Kenyan women, shedding in cervical fluid wasn't significantly linked to spontaneous delivery before 34 weeks.

If you're early in pregnancy or trying to conceive and your swab is positive, don't read it as a miscarriage warning. The evidence can't support that. What matters is blood work that dates your infection, since a positive swab alone can't say whether your infection is new or old.

Passing CMV to a Baby

Congenital CMV is infection passed to the baby before birth. The link between a mother's genital shedding and whether her baby gets infected has been poorly studied, and some babies with congenital CMV were born to mothers in whom no shedding was found.

The biggest fetal risk comes from a first infection during pregnancy. Passing the virus on after a reactivated or new-strain infection is uncommon: in an Italian cohort of women who already carried CMV, none of the 105 with such an infection passed it to the fetus.

After birth, genital fluid seems to matter very little. In a study of term babies followed through their first year, breast milk was the main route of infant infection, the mother's saliva played a minor role, and vaginal secretions played almost none.

So a negative swab doesn't clear a pregnancy, and a positive one doesn't mean your baby is infected. During pregnancy, the fetal question is answered by blood tests that date your infection and, when needed, by testing the fluid around the baby. After birth, a newborn saliva or urine test in the first three weeks of life settles it.

No professional society recommends vaginal CMV swabs for screening. Major obstetric groups such as ACOG and SOGC don't recommend routine CMV blood screening in pregnancy either. Prevention advice centers on hygiene around young children, and diagnosis centers on maternal blood tests when indicated and newborn saliva or urine testing.

Bacterial Vaginosis and Genital Inflammation

Bacterial vaginosis (BV) is a common shift in the vagina's bacteria away from the protective species that normally dominate. In one study, about half of women with BV were shedding CMV in the genital tract, compared with about one in five without it (52% versus 19%), roughly four and a half times the odds. Women with BV who shed CMV were also more likely to carry more than one strain.

Shedding also goes along with local inflammation. Women shedding CMV had higher levels of several of the immune system's inflammatory signaling proteins in cervical and vaginal fluid, a pattern seen in pregnant Kenyan women, in women with HIV, and in the BV study.

If your swab is positive and you've had unusual discharge, odor, or recurring vaginal infections, check for BV at the same time. The two show up together often enough that looking at one without the other leaves an obvious gap.

HIV and a Weakened Immune System

Shedding is far more frequent when the immune system is under strain. Among Ugandan women living with HIV, CMV turned up in vaginal fluid in about four out of five women and at about a third of all clinic visits. Younger women and those with more HIV in their blood before starting treatment shed more often.

In pregnant women with HIV, CMV found in urine (a different sample from this swab) has been tied to passing CMV to the baby. Whether vaginal shedding carries the same signal hasn't been shown directly.

After a First Infection

In healthy women followed after a first CMV infection, vaginal fluid carried more virus than saliva, urine, or blood, and the amount fell over the following months. That makes the genital tract an efficient exit for the virus and a likely route for sexual and birth-related spread.

In adults who've just been infected, shedding into body fluids generally goes on for several months before tapering off. It also comes and goes. In a pilot study that swabbed nine healthy new mothers every day for four weeks, CMV appeared in about 15 of every 100 vaginal swabs.

This is where positive results get misread. The highest amounts of virus appear soon after a first infection, which tempts people to read any positive swab as a new infection. Yet in the studies above, most women with positive swabs had carried CMV for years and were shedding again. The swab answers whether the virus is active now, and your blood antibody tests (covered below) answer when you were first infected, so you need both to know which situation you're in.

When Results Can Be Misleading

• On-and-off shedding: one negative swab can miss someone who sheds intermittently, and one positive swab can catch a short episode that isn't your usual pattern.
• Sample handling: the virus's genetic material breaks down after collection, and storage temperature and the preservative liquid both affect how much is recovered, which pushes errors toward false negatives.
• Swab site: genital CMV seems to come mainly from the cervix, the lower end of the uterus, so a vaginal swab may pick up less virus than a cervical one would.
• Lab method: labs use different assays with different detection limits, so a low-level result at one lab can read negative at another, and occasional contamination between samples can produce a rare false positive.

Tracking Your Trend

Because shedding switches on and off, one swab is a snapshot. Repeats can show whether your genital tract sheds often, rarely, or mainly during certain stretches, such as pregnancy, the months after starting HIV treatment, or a period of heavy contact with young children.

Shedding also seems to climb as pregnancy goes on. In one Italian cohort of women who already carried CMV, vaginal shedding rose from about 13 in 100 women early in pregnancy to about 22 in 100 at delivery.

There are no agreed cutpoints for vaginal CMV and no established retesting schedule. A single result can't change your care on its own, and no study has shown that knowing your shedding pattern improves outcomes for you or a baby. If you do repeat it, treat the series as background context for your blood work.

What to Do With an Unexpected Result

The blood tests that pair with this swab are worth understanding first. IgG is the long-lasting antibody, and a positive IgG shows you've been infected at some point. IgM tends to appear early in a new infection, but it can linger for months and can be triggered by other viruses, so it's unreliable on its own. IgG avidity measures how tightly your IgG binds the virus: low avidity points to an infection in the past two to four months, and high avidity points to an older one.

• Positive swab, not pregnant, feeling well: order CMV IgG and IgM, and add IgG avidity if IgM is positive. A repeat swab in 3 to 6 months can show whether shedding is persistent or occasional, though neither result changes treatment while you're well.
• Positive swab while pregnant or trying: get the same blood work promptly and see an obstetrician or maternal-fetal medicine specialist. Positive IgM with low avidity suggests a recent first infection, which is the situation where valacyclovir, an antiviral started early in pregnancy, has lowered the chance of passing CMV to the fetus in randomized trial data.
• Positive swab with vaginal symptoms: test for bacterial vaginosis and other common genital infections, since BV is linked to more shedding.
• Positive swab with HIV or other immune suppression: involve an infectious disease specialist, who may add a blood test for CMV to check for active infection beyond the genital tract.

A negative swab with a positive IgM, or a positive swab with negative blood work, isn't a contradiction. In pregnant women whose babies were later found to have CMV from a reactivated or new-strain infection, more than half had no active marker in their blood during pregnancy, because the virus can be active in the cervix or other tissues without showing up in blood.