Systematically review risks of an infant being born with low birth weight (LBW), preterm birth (PTB) or small for gestational age (SGA) among married and unmarried women. Medline, Embase, CINAHL, and bibliographies of identified articles were searched for English language studies. Studies reporting birth outcomes of married and unmarried (single and cohabitant) were included. Two reviewers independently collected data and assessed the quality of the studies for biases in sample selection, exposure assessment, confounder, analytical, outcome assessments, and attrition. Meta-analyses were performed using random effect model for both unadjusted and adjusted data and odds ratio (OR), and 95% confidence interval (CI) were calculated. Twenty-one studies of low to moderate risk of bias were included. Compared to married mothers unadjusted odds of (a) LBW was increased among unmarried (OR 1.46, 95%CI 1.25-1.71), single (OR 1.65, 95%CI 1.44-1.88) and cohabitating (OR 1.29, 95%CI 1.25-1.32) mothers; (b) PTB was increased among unmarried (OR 1.22, 95%CI 1.14-1.31), single (OR 1.54, 95%CI 1.39-1.72) and cohabitating (OR 1.15, 95%CI 1.08-1.23) mothers and (c) SGA birth was increased among unmarried (OR 1.45, 95%CI 1.32-1.61), single (OR 1.70, 95%CI 1.47-1.97) and cohabitating (OR 1.36, 95%CI 1.30-1.42) mothers. Meta-analyses of adjusted odds estimates confirmed these findings at marginally lower odds. Maternal unmarried status is associated with an increased risk of LBW, PTB and SGA births.