Issue 177  /  October 6, 2026  /  Feature

Bacterial Vaginosis Comes Back for Half of Women. One Swab Flags Who's at Risk

Women who weren't back to healthy when BV treatment ended were nearly three times as likely to have it again a month later. A second study found obesity leaves lasting marks on a woman's eggs. Both say the most useful test comes at a time doctors don't currently test.

Bacterial Vaginosis Comes Back for Half of Women. One Swab Flags Who's at Risk

Researchers at Monash University and Bayside Health's Melbourne Sexual Health Centre combined data from 188 heterosexual couples across four Australian trials run between 2015 and 2025.

Every woman had bacterial vaginosis (BV), the most common vaginal condition in women of reproductive age, and took the standard week of antibiotics. Every male partner was treated at the same time, with antibiotic pills and an antibiotic cream applied to the penis twice a day for seven days. Women gave a vaginal swab on day 8, the day after treatment ended and before couples had sex again, and a second swab four weeks later.

Lenka Vodstrcil led the study, with Catriona Bradshaw as senior author. It was published online September 30 in The Lancet Obstetrics, Gynaecology & Women's Health.

Treating both partners largely removed the chance of catching BV back from the partner, so the day 8 swab mostly reflects how well the antibiotics themselves worked. On day 8, 65 of 176 women with a usable swab (37%) were not yet back to a healthy balance of vaginal bacteria, based on a standard lab score read under the microscope. Four weeks later, 22 of those 65 women (33.8%) had BV again, compared with 14 of the 111 women whose swabs were healthy on day 8 (12.6%). After accounting for differences between the trials, women with an abnormal day 8 swab were 2.8 times as likely to have BV again. The researchers combined earlier trials after the fact, and only one of the four came from a randomized trial.

They report a link, not proof of cause, and every couple was monogamous and recruited in urban Australia, mostly at one clinic.

Two days later, Nature Metabolism published a study from a team led by Qiang Wang at Nanjing Medical University.

In mice, obesity in the mother changed the chemical tags that switch genes on and off in her eggs, at genes involved in metabolism. An embryo normally wipes those tags clean as it develops. These were wiped, yet a second marker stayed in place at the same spots, and the changed tags showed up again in the offspring. When the team added the tags to mouse eggs on purpose, the offspring had problems with blood sugar, mostly the females.

Eggs from women with obesity carried the same tags at two of those genes. The cause and effect was shown only in mice. In women, the study shows the tags exist in the eggs, not that they affect a child's health.

The two studies come from different fields and find the same thing. Something that was supposed to be gone was still there, and the moment that predicts what happens next falls outside the doctor visits care is built around. For BV, it is the day after antibiotics end, when US guidelines don't ask for a check. For a child's metabolic health, at least in mice, it is before conception, months ahead of the first prenatal visit.

That is where women's health diagnostics can grow, and investors are already paying for it.

Companies that sell vaginal microbiome tests raised $73 million between them in the past three weeks, and the larger round names fertility as its first new market.

US guidance does not ask for a test the day after BV treatment. The Centers for Disease Control and Prevention's 2021 treatment guidelines, still the main US reference, say follow-up visits are unnecessary if symptoms go away. The same guidelines say partners don't need routine treatment.

The American College of Obstetricians and Gynecologists moved past that last year, when it began recommending that doctors consider treating the male partner at the same time for some women whose BV keeps coming back.

That change followed the same Monash team's 2025 trial in the New England Journal of Medicine. Within 12 weeks, 24 of 69 women whose partners were treated (35%) had BV again, compared with 43 of 68 women whose partners were not (63%). Treating the partner cut repeat BV sharply and still left about a third of women with it again.

The new study points to why. Treating the partner stops women catching BV back from him. It does nothing for bacteria that survived the woman's own antibiotics. The authors write that these women will likely need stronger treatment of their own, and Bradshaw's team is now testing longer courses and new combinations of antibiotics.

They suggest a check-up swab after antibiotics and before couples have sex again, which they say could pick out the women most likely to get BV back.

Among women who used an IUD and still had an abnormal swab on day 8, 8 of 17 (47.1%) had BV again at four weeks, compared with 7 of 72 (9.7%) among women with neither. The group is small, and IUD use on its own was not a clear predictor once the researchers accounted for other factors.

Most of the IUD users, 45 of 57, had a hormonal IUD rather than a copper one, and the authors say more research is needed before drawing conclusions.

For companies selling vaginal microbiome tests, the study supports a second test after treatment, something guidelines don't currently include.

It also shows the useful signal came from an old, simple lab score, one of the traditional methods the CDC notes costs less than newer DNA-based tests.

The microscope score sees the shapes of bacteria, not their exact species. Lactobacillus, the main healthy bacteria in the vagina, comes in several types, and one that often appears right after treatment can be a sign the balance won't hold.

Under the microscope, it looks the same as the protective types. The team plans DNA sequencing work to tell them apart.

That is what DNA sequencing tests sell. Evvy announced a $40 million funding round led by Catalio Capital Management on September 15 to test its AI-based diagnostics in clinical studies, starting with fertility, and CEO Priyanka Jain told Fierce Healthcare the company is working toward insurance coverage. Evvy says it has served more than 100,000 patients. On September 29, Tiny Health, which also sells vaginal microbiome tests, announced a $33 million round led by B Capital.

The egg study points to the same patients Evvy has named as its next market, women preparing to conceive. The Nanjing team writes that its findings point to the months before conception as the time to step in, and fertility clinics already see many women in that period.

The question that would turn a mouse finding into something doctors can use is whether those tags on the eggs fade when a woman's metabolic health improves before she conceives.

Diagnostic money has gone into the test at the first visit. The past week's research says the answers sit before and after it.

This is market and evidence analysis, not medical advice. Whether any test or therapy is appropriate for an individual is a clinical decision.

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Sources

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Institutional (press release)

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— Keisha, Editor of The Luteal