Background: The United States has a relatively high preterm birth rate compared with other developed nations. Before the enactment of the Affordable Care Act in 2010, many women at risk of a preterm birth were not able to access affordable health insurance or a wide array of preventive and maternity care services needed before, during, and after pregnancy. The various health insurance market reforms and coverage expansions contained in the Affordable Care Act sought in part to address these problems. This analysis aims to describe changes in the patterns of payer mix of preterm births in the context of a post-Affordable Care Act insurance market, explore possible factors for the observed changes, and discuss some of the implications for the Medicaid program. Methods: We applied a repeated cross-sectional study design to explore payment mix patterns of all births and preterm births between 2011 and 2016, using publicly available National Vital Statistics Birth Data. We included an equal number of years with payment source available in the dataset before and after January 1, 2014, when the coverage expansions became effective. Results: We found a small relative change in payment mix during the study period. Private health insurance (PHI) paid for a higher percentage of all births and this rate increased steadily between 2011 and 2016. Preterm births paid by PHI increased by 1.4 percentage points between 2011 and 2016 and self-pay/uninsured preterm births decreased by 0.3 percentage points over the same time period. Medicaid had the highest, and a relatively stable, preterm birth coverage percentage (48.9% in 2011, 49.2% in 2014, and 48.9% in 2016). Medicaid was also more likely to pay for preterm births than PHI, but this likelihood decreased by more than one-half after 2014 (8.2% in 2013 vs. 3.8% in 2014). Conclusions: After the 2010 reforms, Medicaid remained a constant source of coverage for the most vulnerable women in society when faced with the high cost of a preterm birth. Nationwide, of the 64 million women ages 15 to 44, 4% gained PHI (directly purchased or employer sponsored) and another 4% Medicaid, with a concomitant 8% decrease in uninsured women of reproductive age between 2013 and 2017. More research is needed to conclude with certainty that the reforms worked as intended, but the important role of Medicaid as a financial safety net is undeniable.