This study will include 402 participants in Belgium, and the study will be divided into two phases. The study starts with a phase A (= internal pilot study) in which approximately 154 patients will participate. In the internal pilot study, we will further investigate the effect of the oral synbiotic on the vaginal microbiome, so that we will schedule an extra hospital visit.

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If the results of the pilot study are positive (ie if the oral synbiotic has been shown to be effectively present in the vagina), phase B will start for the remaining number of patients. When you participate in phase A and the results of the internal pilot study are negative, the study and the intake of the synbiotic will be stopped. This can happen in the middle of your pregnancy, but will not affect standard care and follow-up of the pregnancy.

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The study consists of three periods:

• The start period: here we check whether you can participate in the study, vaginal swabs are taken and you will be assigned to either the group that receives the oral synbiotic or the placebo. You then also start taking it.

• The intervention period: starts from the first dose of the oral synbiotic or placebo and runs until your delivery.

• The neonatal follow-up period: when your baby is admitted to a neonatal intensive care unit (NICU) in the first 7 days, we will monitor your baby until discharge from the hospital.