Vaginal Dysbiosis

Preterm birth is a major cause of mortality and long-term disability. Bacterial vaginosis (BV) is a frequent form of dysbiosis that is often asymptomatic and increases the risk of preterm birth. BV is usually diagnosed using conventional tools such as the Nugent score. Molecular diagnosis of BV has now been shown to be more reproducible and to provide a more accurate characterization of dysbiosis. The main objective of this study is to evaluate the effectiveness of a self screen and treat strategy for vaginal flora dysbiosis, based on molecular point of care (POC) multiplex technology before 18 weeks' gestation, in reducing the rate of preterm birth among pregnant women at high risk, compared with usual care. The hypothesis is that a Screen-and-Treat strategy using molecular biology among women with previous preterm or/and an history of late abortion could be effective in reducing preterm births by 40%.

The goal of this clinical trial is to evaluate FB301 during a mock frozen embryo transfer (FET) cycle in pre-menopausal women aged 18 to 40 years with a prior failed FET and a defined vaginal bacterial imbalance (dysbiosis). The main question it aims to answer is: • Does treatment with FB301 during a mock FET cycle change the proportion of participants who meet the predefined vaginal bacterial threshold compared with placebo? Researchers will compare: * FB301 given after an initial vaginal cleansing with chlorhexidine (an antiseptic solution), * FB301 given after an initial vaginal cleansing with saline (saltwater), and * A placebo capsule given after an initial vaginal cleansing with saline to determine whether these approaches affect vaginal bacterial composition and pregnancy outcomes. Participants will take study treatment for 15 days during a "mock" FET cycle. In a mock cycle, participants receive the hormones needed to prepare the uterus for embryo transfer, but no embryo is transferred. After completing the mock cycle, participants will proceed with their planned frozen embryo transfer and will be followed during pregnancy until birth. Participants will: * Undergo a vaginal cleansing before receiving the first dose of FB301 or placebo * Provide vaginal swab samples at up to 5 study visits * Attend the study centre for up to 8 visits and participate in up to 4 follow-up phone calls * Complete a mock FET cycle before proceeding with their planned frozen embryo transfer cycle Participants who become pregnant will be followed until birth.

The hypothesis is raised of improving the status of vagnal microbiota with a non-hormonal topical product in early situations of dysbiosis in which the vaginal pH is elevated and the patient may or may not be symptomatic, in order to try to prevent the appearance of future established vaginal infections.