A three-period crossover trial will study the effects of short-term use of 3 menstrual products (menstrual pad, tampon, menstrual cup) on the bacterial composition of the vaginal microbiome in three countries (Peru, Cameroon, and Switzerland). Each crossover period consists of two menstrual cycles, thus resulting in a 6-month trial. Participants will be randomly assigned to one of 6 exposure sequences (different order of products per sequence); in each sequence, participants will adopt each menstrual product for 2 menstrual cycles. Participants will provide vaginal microbiome samples via self-sample swabs at 3 points during each menstrual cycle. Given the crossover design of the study, there is not a dedicated control group.

In three countries, the investigators will follow a randomized crossover design to study the effects of menstrual products on the vaginal microbiome composition and diversity. Eligible participants who are willing to adopt pads, tampons and menstrual cups to manage their menstruation, and for which baseline composition of their vaginal microbiomes is determined through DNA sequencing of the 16s rRNA gene will enter a three-period crossover trial. Each period of the trial will consist of two menstrual cycles, thus resulting in a 6-month trial. Participants will be randomly assigned to one of six different exposure sequences of menstrual products, as follows:

i) Tampon, Cup, Pad ii) Tampon, Pad, Cup iii) Cup, Tampon, Pad iv) Cup, Pad, Tampon v) Pad, Tampon, Cup vi) Pad, Cup, Tampon

In group "i)", participants will adopt tampons during the first two menstrual cycles (first period of the trial), followed by the adoption of cups during the next two menstrual cycles (second period), and lastly menstrual pads for the next two menstrual cycles (third crossover period). In groups ii) to vi) following the same design but in different sequences. Self-sampling kits will be provided during six consecutive menstrual cycles, a few days before the start of menses. Each kit will contain swabs for three self-sampling points during the menstrual cycle and the corresponding menstrual products.

Inclusion Criteria:

• able to provide informed consent;
• aged 18-35;
• residing in the recruitment region and expecting to reside in the same region for at least six months following recruitment (Yaoundé/ Gounougou, Cameroon; Loreto/Lima/Maynas provinces, Peru; or Basel-Landschaft and Basel-Stadt cantons, Switzerland);
• are native speakers or are able to read and understand one of the following languages in each respective country (German, French, Italian, English or Spanish in Switzerland; French or English in Cameroon; Spanish in Peru);
• had menstrual cycles of 21-35 days for at least the last 4 months;
• had menses which lasted at least 3 days for at least the last 4 months.

Exclusion criteria:

• experienced a menstrual abnormality with any of the menstrual cycles in the last 4 months (such as oligomenorrhea or amenorrhea, or bleed more than 7 days a month);
• are pregnant or actively trying to become pregnant;
• are breastfeeding;
• used antibiotics and/or vaginal antifungals in the last 30 days prior to recruitment;
• had a vaginal birth in the last 6 months;
• vaginal surgery, perineal surgery, uterine surgery, miscarriage or abortion in the last 6 months;
• history of Toxic Shock Syndrome ;
• positive to detection of the toxic shock syndrome toxin-1 gen (tst) using Polymerase Chain Reaction (PCR);
• Intrauterine device in situ;
• under medication (treatment against infectious or chronic disease) at the time of recruitment or during the 3 weeks prior to recruitment;
• clinical symptoms of vaginal infection;
• smoker.