OBJECTIVE: The incidence of preterm labor is increasing and continues to be a significant cause of neonatal mortality and morbidity. Various risk factors of preterm labor are known, among those a short interval between labor and a subsequent conception. The risk of spontaneous preterm labor increases with interpregnancy intervals shorter than 12 months, the strongest association with intervals shorter than 6 months. Cervix uteri consist predominantly of fibrous connective tissue and it is from this tissue cervix derives its "strength". A prerequisite for vaginal delivery is a decrease in the cervical collagen concentration. The aim of this study is to describe the normalization of the cervical collagen after labor. METHODS: Cervical biopsies were collected 3, 6, 9, 12, and 15 months after labor from 15 women. Collagen concentrations of the biopsies were measured following a standard Hydroxyproline assay. The repeated measures of collagen concentration after labor were analyzed by a one-way analysis of variance. RESULTS: Compared with collagen 3 months after labor the collagen concentration was still increasing until 12 months after labor. Statistical significant difference were found in the collagen concentration 3, 6, 9, and 12 months after labor (50.2 (48.0;52.3), 57.9 (55.7;60.0), 61.9 (60.1;63.8) and 65.2(63.4;67.0) µg/mg dry weight; p<0.01). No statistically significant difference was found between the collagen concentration 12 and 15 months after labor. CONCLUSIONS: The human uterine cervix has not normalized until 12 months after labor. The lower collagen concentrations may explain the association between short interpregnancy interval and preterm labor.