Background Low birth weight is a significant risk factor for neonatal morbidity and mortality as well as a general indicator of the health status of a population. Objectives: To determine the influence of socio-demographic, obstetric and prenatal care variables in low birth weight.

Methods This is a cross-sectional, descriptive, correlational and prospective study. The sample is probabilistic intentional, consisting of 1846 mothers, of which 161 had newborns with low birth weight. A questionnaire was used to collect data between March 2010 and May 2012 in 26 Portuguese public health institutions.

Results In this study, low birth weight was associated to: absence of pathologies during pregnancy, absence of gestational age risk, absence of obstetric risk, maternal age considered of risk or not/absence of gestational age risk (p = 0.000), low education/monthly income level ≤ 1000 €, low education/unemployment, number of children of risk/absence of gestational age risk, no consumption of alcohol and tobacco/absence of pathologies during pregnancy, no consumption of alcohol and tobacco/absence of gestational age risk pathologies during pregnancy/absence of gestational age risk, presence of previous pathologies/absence of gestational age risk, twin pregnancy/absence of gestational age risk, and with an error of 10% to low education level, unplanned pregnancy, late onset of prenatal surveillance, and absence of previous pathologies.

Conclusion The determination of the factors that favour the development of low birth weight is of fundamental importance to the understanding, planning and development of actions within mother-child health.