Poor pregnancy outcomes such as prematurity, low birth weight and infant mortality are societal indicators of a nation’s health status. These indicators have remained at exceptionally high rates in the United States despite the levels of resources and technology. In the quest to understand that discrepancy, among the ranges of theories and models for explaining poor pregnancy outcomes an emerging concept is coming to attention: social capital. In order to test whether maternal social capital has an impact on pregnancy outcome, women in a Healthy Start program were surveyed over a 13-month period to assess their social capital levels and then their pregnancy outcomes. What emerged was that maternal social capital can predict up to 47% of the variance in pregnancy outcome. That is a powerful research result considering that previously there has been no literature tracing a link between maternal social capital and pregnancy outcome. In this study, maternal risk factors adversely affect up to 30% of the variance in pregnancy outcomes. Previous research has focused on maternal risk factors as the primary reason for high rates of preterm delivery, low birth weight, and infant mortality in the United States. However, this research found that in the sample of women at risk for adverse pregnancy outcomes, maternal risk factors had a very strong influence on maternal social capital (R-square=65%) while their effects on pregnancy outcomes were about half of their effects on social capital. This result suggests that social capital mediates the effects of maternal risk factors on pregnancy outcomes. It appears that one of the reasons that the high rates of adverse pregnancy outcomes in the United States have remained a mystery is that maternal social capital has not been taken into account.