In 1999, birth defects were the leading cause of infant mortality, accounting for 19.7 percent of infant deaths, followed closely by prematurity/low birthweight (15.7 percent).  Among deaths due to birth defects, the largest proportion were due to heart defects, accounting for nearly 1 out of 3 birth defects-related infant deaths (29 percent).  The second leading fatal type of birth defect in 1999 was chromosomal defects (17.4 percent), followed by defects of the nervous system (12.6 percent) and respiratory defects (10.4 percent).  The “other” birth defects category (11.0 percent of birth defects-related infant deaths) combines all remaining types of fatal birth defects.