The purpose of this study was to determine the factors affecting low birth weight (LBW) at four central hospitals in Vientiane, the capital city of Lao PDR. We collected data from 235 mothers with LBW babies and 265 with babies of normal birth weight (NBW) who had delivered them at four central hospitals from March to June of 2008. Among the mothers of LBW babies, 84.7% were aged 18-35 years, 7.7% aged < 18 years, and another 7.7% aged > 35 years, while the corresponding figures among those with NBW babies were 97.7%, 1.1%, and 1.1%, respectively. The mothers in a low income group earning < 1 million Kips (1 USD = 8,700 Kips) per month numbered 68.9% for the LBW group and 15.1% for the NBW group. We estimated both odds ratios (ORs) and 95% confidence intervals (CIs) to explore associations between LBW outcomes and related factors. Significant associations with LBW were found for delivery at age < 18 years (OR = 8.6, 95% CI = 2.4-30.7), monthly family income < 1 million Kips (OR = 13.9, 95% CI = 8.8-21.9), physical labor during pregnancy (OR = 5.0, 95% CI = 3.1-8.1), and first child birth (OR = 2.2, 95% CI = 1.5-3.3). Mothers with an inadequate level of knowledge regarding healthy pregnancy practices were at 10.1 times higher risk of developing LBW babies (95% CI = 6.7-15.2). Mothers lacking adequate nutritional practices were more prone to deliver LBW babies with an OR of 8.9 (95% CI = 5.6-14.3). The ORs of LBW babies for those factors were high among Lao mothers. Improving a mother's knowledge of and practice for a healthy pregnancy needs to be emphasized to reverse these LBW trends.