BACKGROUND The study aims at confirming or rejecting the hypothesis of an association between birthweight and post partum uterine infection. METHODS A case-referent study was performed on 51 puerperal women with clinical signs of endometritis-myometritis. To each case an otherwise healthy puerperal woman was recruited and matched for age, parity and days after delivery. RESULTS The proportion of women having had newborns with birthweight < 2500 g was 20/49 among cases and 2/50 among referents (odds ratio 16.6; 95% CI 3.5-152.3). Preterm births were registered in 15/50 cases and 2/49 referents (odds ratio 10.1; 95% CI 2.1-94.5). The average gestational age at delivery was approximately 2 weeks shorter among cases than among referents (37.5 versus 39.5 weeks). CONCLUSIONS Low birth weight was ten times more prevalent among women with puerperal infection than among healthy puerperal women. The findings indicate that giving birth to a low birth weight baby is strongly associated with ensuing puerperal infection, possibly by a subclinical antenatal intrauterine infection, predisposing to both adverse fetal and maternal outcomes of pregnancy.