Medicaid expansion drove revenue growth at insurers in the first quarter, and Centene's (NYSE:CNC) shares have rallied to new highs on expectations that growth continued through the second quarter.

Over 6 million people have signed up for Medicaid coverage since the Affordable Care Act launched last fall, providing signficant support for companies such as Centene, Molina (NYSE:MOH) and Health Net (UNKNOWN:HNT.DL) that run state programs.

Source: Centene Corp.

In the following slideshow, I address Centene's exposure to Medicaid enrollment growth and provide insight into key considerations investors should keep in mind when Centene reports earnings on July 22.