Seventy-eight percent of U.S. mothers begin prenatal care during the first three months of pregnancy; 18 percent wait until the second three months; and five percent wait until the third trimester or receive no care at all. Patterns of prenatal care vary widely among population subgroups: Mothers younger than 18 and unmarried mothers are the least likely to obtain first-trimester care (49 percent and 56 percent, respectively), and the most likely to obtain care only in the third trimester or none at all (about 12 percent of each group). Women aged 18-19, blacks, Hispanics, poor women and women with little education also have disproportionately high levels of very late or no care (7-9 percent). Married, white, nonpoor women, in contrast, obtain the most timely prenatal care: In 1980, only two percent initiated care in the third trimester or received no care. Compared with this subgroup of women, the population as a whole has two times the risk of obtaining inadequate care. Unmarried women run the highest relative risk (five times the risk for married, white, nonpoor women), followed by teenagers, Hispanic women, women with little education, poor women and blacks (who have from three to more than four times the risk of the comparison group).