We conducted a retrospective study to determine the association between maternal body mass index and pregnancy weight gain with low birth weight newborns (LBWN) at Phrapokklao Hospital in eastern Thailand. We evaluated the files of 2,012 women who delivered at the hospital. Data obtained from the charts were parity, maternal age, body mass index (BMI), prepregnancy weight, weight gained during pregnancy, gestational age, hematocrit level, referral status, place of residence, fetal presentation, completion of antenatal care visits and maternal HIV infection. Sixty-five point two percent of subjects were aged 20-34 years old. Fifty-seven percent of subjects had a normal BMI and 13.2% were anemic. Thirty- seven point five percent, 32.9% and 29.6% gained too little, the correct amount and too much weight during pregnancy, respectively. Primiparity, too little weight gain and gestational age less than 37 weeks at delivery were all significantly associated with LBWN. Preterm babies were 25 times more likely to have a low birth weight than term infants (adjusted OR = 24.995; 95% CI: 16.824-37.133, p < 0.001). When maternal weight gain of any BMI group was inadequate, the subject had a 3.4 times greater risk (adjusted OR = 3.357; 95% CI: 22.114-5.332, p < 0.001) of having a LBWN. Primiparous women had a 1.7 times (adjusted OR=1.720; 95% CI: 1.182-2.503, p-0.005) greater risk of having a LBWN. The results from this study may be useful to plan maternal health programs for eastern Thailand.