BACKGROUND: The 1997 State Children's Health Insurance Program (SCHIP) program allowed states to expand Medicaid to uninsured children through age 18 in families under 200% of the federal poverty level. Prepregnancy insurance coverage of adolescents may help reduce unintended pregnancies, address other medical issues, and allow for early and adequate prenatal care for those carrying to term. OBJECTIVES: We tested the effects of SCHIP implementation on insurance coverage for teenage mothers and investigated whether these effects varied by type of state SCHIP program--Medicaid expansion, stand-alone program, or some combination of these. RESEARCH DESIGN: We used Pregnancy Risk Assessment Monitoring System data from 1996 through 2000 and difference-in-differences analysis to analyze coverage changes for teenage mothers (age