Background: Having a low birth weight (LBW) baby can cause emotional, social and financial stress for the family. Settings and Design: A longitudinal study of 1 year duration (June 2009-May 2010) was conducted in an urban slum of Mumbai. Universal sampling method was employed and every antenatal women registered at the urban health center from June 2009 to August 2009 were included as study participants. Materials and Methods: Of the 231 pregnant women that registered, 21 were excluded from analysis as - 5 home deliveries, 3 still births, 3 spontaneous abortions, and 10 that could not be traced. Thus for final analysis, sample size was 210. Inclusion and Exclusion criteria were: All antenatal care (ANC) subjects registered at the urban health center within 20 weeks of their gestational age were included as the study subjects. Subjects with only live hospital/institution birth were included. These registered women were then followed-up for next 9 months for monitoring maternal weight gain and neonatal birth weight. Statistical Analysis: Statistical analysis was done using SPSS version 17. Chi-square test was used for testing significance of association at P value of 0.05 and 0.001. Results: Proportion of LBW in the present study is 27.1% with mean birth weight 2.2285 kg with S.D. 0.2281 kg. Factors like maternal age, education, tobacco addiction, hemoglobin levels, gestational age, and interval between pregnancies were significantly associated with birth weight. Conclusions: LBW continues to be a significant public health problem and as multiple factors are associated with it, it requires a more holistic and multipronged approach for its reduction.