The objective of the project is to elucidate if the oral administration of Ligilactobacillus salivarius CECT5713 is capable of improving the rates of term pregnancies in couples with infertility problems of unknown cause. On the other hand, the project is aimed to detect microbiological, immunological and hormonal markers that allow the identification of couples in which the administration of the strain could be particularly effective. This is a randomized, double-blind, parallel-group controlled nutritional intervention study. The intervention (Ligilactobacillus salivarius CECT5713 or placebo) will be performed during the 6 months prior to the fertility treatment and during the first IVF cycle (estimated 1-2 months). In case of pregnancy, the woman will continue the treatment until 12 weeks of gestation.

Inclusion Criteria:

• legal age
• Intention to achieve a pregnancy, but without achieving it;
• Be willing to undergo in vitro fertilization treatment;
• Be on the waiting list for the IVF cycle with an expected waiting time > 5 months.

Exclusion Criteria:

• Sterility in one of the members of the couple (azoospermia; impermeability of the fallopian tubes...).
• Genitourinary malformations;
• Concurrence of other severe diseases (e.g.: cancer, AIDS, ALS, morbid obesity...) or uncontrolled diseases (inflammatory bowel diseases, diabetes...).
• Treatment/intervention other than the planned IVF after the start date of the intervention.
• Antibiotic treatment at the start date of the intervention.
• Intention to consume another probiotic supplement in the following 3 months.
• Participation in another clinical trial.
• Allergic/intolerant to the excipient.
• Inability to understand the informed consent form and/or to follow the basic instructions of the trial.

The target is infertility of unknown origin. The outcome is a successful pregnancy.

Global and individual reproductive rates, microbiological and immunological data will be available to other researchers once the study (including analyses of all the samples) is completed and published.

All the raw data will be available to other researchers once the study is published.

Our intention is that all the raw data will be available as a supplementary tables in an open acess journal. They will be also provided on request.

Triple (Participant, Care Provider, Investigator) Only one person of the collaborator that provides the capsules (Biosearch Life) know the correspondecs between each reference (one reference per each subject) and the arm (probiotic or placebo). The codes are blinded for the participant, the care provider and the investigator.