We sought to examine the health status of disadvantaged pregnant women more broadly and to consider if poor maternal health predisposes a woman to an adverse birth outcome. We surveyed 250 women recruited from six health centers in the greater Boston area during their third trimester. The main predictor variables were maternal physical functioning (PF), emotional health (EH), and overall health status in the month prior to pregnancy. The main outcome variables were the decline of maternal PF and EH during pregnancy and adverse birth outcomes. Mean PF scores fell from 91.9 prior to pregnancy to 63.7 during the third trimester (mean scores transformed 0 to 100, where a higher score represents better health). EH remained unchanged during pregnancy. After adjustment, women with a preexisting medical condition reported a lower PF score prior to pregnancy (87.8 versus 94.5, p < 0.05). Poor PF prior to pregnancy or during the third trimester was associated with an increased risk of preterm labor (odds ratio 2.02, 95% confidence interval 1.03-3.97). This study is the first to employ general health status measures to examine changes in health during pregnancy. Our findings support the use of preconception care to improve the health status of disadvantaged women with pre-existing conditions. This study suggests that poor maternal health may predispose a woman to an increased risk of preterm labor.