In a recent effort to lower the US infant mortality rate, Congress has expanded the Medicaid coverage options that states may offer pregnant women. Careful evaluation of changes in perinatal outcome associated with this expanded coverage is needed. The linkage of Medicaid enrollment files of mothers and infants to birth, death, and fetal death certificates is an initial step in assessing the effectiveness that Medicaid coverage expansions have had on pregnancy outcome. Creation of such a database for Tennessee for 1984-1987 revealed that complete information on mother, delivery, and child is available for only three quarters of Medicaid-reimbursed births. Furthermore, Medicaid-reimbursed births that had all three data components had different characteristics and lower mortality rates than did births with missing elements. Those persons seeking to evaluate expanded Medicaid coverage for pregnant women need to be aware that consideration of only those births for whom there is information on mother, delivery, and child may lead to serious underascertainment of fetal, perinatal, and neonatal mortality rates.