Abstract Objectives: to investigate the influence of social and health factors on low birth weight (LBW) among Polish women. LBW is defined as birth weight less than 2500g. Materials: 27 015 perinatal data gathered from 40 Polish hospitals taking part in the OBSQID international project. Methods: Descriptive statistics and multifactorial logistic regression. Dependent variable: LBW. Independent variables: maternal age, place of residence, marital status, education, parity, smoking, diseases before and during pregnancy, obstetrical history, pregnancy weight gain rate, prepregnancy BMI. Results: 6,4% LBW in all neonates, 2,0% LBW in full term neonates were ascertained. Among vaginal deliveries, increased risk of LBW have: underweight (OR=2,2); small pregnancy weight gain rate (OR=2,2); unmarried (OR=1,9); multiparous (OR=1,5); elementary education (OR= 2,7); professional education (OR=2,3), mother age less than 20 years (OR=1,5); smoking before pregnancy (OR=1,7). Among cesarean section deliveries increased odds have: small pregnancy weight gain rate (OR=2,9), residence in town (OR=2,0), elementary education (OR= 4,4); professional education (OR=2,8). LBW odds ratio for small pregnancy weight gain rate jointly with prepregnancy low BMI due to other factors was considerably high (OR=7,1 for vaginal delivery, OR =2,6 for cesarean section). Conclusions: prepregnancy low BMI together with small pregnancy weight gain rate is an important risk factor for LBW. Mother age (under 20 years of age) decreases the risk of LBW.