Abstract Design: The following study is an attempt to determine whether and to what extent body mass at birth in premature infants is conditioned by some specific demographic and medical factors. Materials and Methods: The following investigation tools have been applied: standardized EUROPOP GROUP poll questionnaire with our own modifications and adjustments, corresponding to Polish conditions, as well as a poll questionnaire designed by the authors of the following work investigating women’s lifestyle during pregnancy, and medical records analysis. The study group consisted of 555 respondents, whose pregnancy ended between 22 and 36 completed weeks’ gestation, calculated from the first day of the last menstrual bleeding. Results: Statistical analysis of data confirmed the relationship between body mass in premature infants and family income (p<0.0001). Younger women more often (p = 0.01) gave birth to children with extremely low birth weight. Similarly, low-educated women with basic qualification and training, gave birth to children with extremely low birth weight far more frequently (p = 0.04) than well-educated women. The obtained results show that birth weight of the newborns also depended (0.02) on the time of the first contact with the doctor during pregnancy. Conclusions: Body mass in premature infants proved to be conditioned by the analyzed demographic factors. Young maternal age, poor education and low family income had a significant effect on giving birth to an infant with extremely low body mass. Giving birth to an infant with the body mass between 500-999g was significantly related to the fact that the mother reported late for the first antenatal check up (after first trimester). Furthermore, the obtained results allow us to conclude that short intervals between pregnancies (less than 2 years) have unfavorable effect on the body mass in premature infants.