Updated project metadata. Updated publication reference for PubMed record(s): 29425317. Preterm birth is a leading cause of neonatal mortality and lacks an effective therapy. Ascending microbial infections from the lower genital tract lead to infection of the placenta, amniotic fluid and fetus causing preterm birth or stillbirth. Directly in the path of an ascending infection is the cervical mucus plug (CMP), a dense mucoid structure in the cervical canal with potential antimicrobial properties. In this study, we aimed to define the components of CMP responsible for antimicrobial activity against a common lower genital tract organism associated with preterm birth and stillbirths, namely Group B Streptococcus (GBS). Using a quantitative proteomic approach, we identified antimicrobial factors in CMPs that were collected from healthy human pregnancies. This study aims to provide new insight into how the human CMP may limit ascending bacterial infection.