It is recognized that prenatal care plays an important role in reducing adverse birth.Chinese pregnant women with medical condition were required to seek additional health care based on the recommended at least 5 times health care visits.This study was to estimate the association between prenatal care utilization (PCU) and preterm birth (PTB),and to investigate if medical conditions during pregnancy modified the association.This population-based case control study sampled women with PTB as cases;one control for each case was randomly selected from women with term births.The Electronic Perinatal Health Care Information System (EPHCIS) and a questionnaire were used for data collection.The PCU was measured by a renewed Prenatal Care Utilization (APNCU) index.Logistic regression models were used to estimate odds ratios (OR) and the 95％ confidence interval (95％ CI).Totally,2393 women with PTBs and 4263 women with term births were collected.In this study,695 (10.5％) women experienced inadequate prenatal care,and 5131 (77.1％) received adequate plus prenatal care.Inadequate PCU was associated with PTB (adjusted OR:1.41,95％ CI:1.32-1.84);the similar positive association was found between adequate plus PCU and PTB.Among women with medical conditions,these associations still existed;but among women without medical conditions,the association between inadequate PCU and PTB disappeared.Our data suggests that women receiving inappropriate PCU are at an increased risk of having PTB,but it does depend on whether the woman has a medical condition during pregnancy.