Inclusion Criteria:
1. Able to understand the study and sign a consent form.
2. Is a woman aged 18 to 45 years.
3. Still has regular menstrual periods (pre-menopausal).
4. Has had at least one previous frozen embryo transfer (FET) that did not result in pregnancy within the past 3 years, and this was not due to either male infertility or blocked /damaged fallopian tubes.
5. Willing to take part in mock frozen embryo transfer (FET) cycle and is considered medically suitable to subsequently have a frozen embryo transfer (FET) using one good-quality embryo (grade 3BB or higher).
6. Willing to have only one embryo transferred during the study cycle.
7. A vaginal swab taken at the first study visit shows a specific vaginal microbiome imbalance required for this study (vaginal molecular dysbiosis).
8. An ultrasound scan shows no problems with uterus or pelvis that would prevent pregnancy or greatly reduce the chance of pregnancy.
9. Recent fertility blood tests show FSH less than 12 IU/L (within the last 6 months)
10. Recent fertility blood tests show AMH greater than 1 ng/mL (at the time of oocyte retrieval).
11. Body mass index (BMI) is between 18 and 42 kg/m².
12. From the first study treatment until the mock FET cycle, agrees to:
- Avoid vaginal intercourse or use condoms (without spermicide or lubricant),
- Avoid swimming or hot tubs,
- Avoid using tampons, menstrual cups, sex toys, vaginal cleansers, lubricants, or other vaginal products unless approved by the study team.
13. Willing to answer questions about sexual and reproductive activity during the study.
14. Willing to provide vaginal swab samples at the clinic.
15. Willing to receive the first study treatment in the clinic and use the study treatment at home as instructed.
16. Screening blood tests are negative for hepatitis B, hepatitis C, HIV, and syphilis.
17. Willing to attend all study visits and follow-up assessments and provide information about pregnancy and newborn health outcomes.
Exclusion Criteria:
1. History of five or more failed frozen embryo transfers (FET) in the past 5 years.
2. Have a condition that weakens the immune system (such as uncontrolled diabetes, active cancer, or an organ transplant).
3. Currently taking medications that suppress the immune system (such as systemic steroids, biologics, chemotherapy, or transplant medications).
4. Is already pregnant before completing the mock FET cycle.
5. Test positive for a sexually transmitted infection (such as chlamydia, gonorrhea, Mycoplasma genitalium, or Trichomonas vaginalis).
6. Current vaginal yeast infection that requires treatment.
7. Cannot safely take the hormone medications required for a frozen embryo transfer (such as estradiol or progesterone).
8. Use of intravaginal progesterone gels or solutions.
9. Needs regular antibiotics or are likely to require antibiotics during the study.
10. Has taken oral, injectable, or vaginal antibiotics within 30 days before screening.
11. History of gynecological cancer or other significant gynecological condition that would make participation unsafe.
12. Has a medical condition that would make participation unsafe or difficult.
13. Has an unstable medical or psychiatric condition that could make it difficult to follow study requirements.
14. Has any other condition that could interfere with participation.