The National Maternal and Infant Health Survey provides new data on the prevalence of unintended childbearing in the United States: Thirty-six percent of births in 1988 were mistimed and 7% were unwanted, while 57% were intended. Although the level of unintended childbearing is high in almost all socioeconomic subgroups of women, the proportion of births that were mistimed or unwanted was 50% or more among age-groups 15-17 (78%), 18-19 (68%) and 20-24 (50%), and among never-married women (73%), formerly married women (62%), black women (66%), women living below the federal poverty level (64%) or at 100-149% of the poverty level (52%), women with less than 12 years of education (58%) and women who already had two children (53%) or three or more children (60%). Multivariate analyses indicate that births to unmarried women--whether formerly married or never-married--are less likely than those to married women to be wanted and more likely to be mistimed. Poverty status has no independent effect on the odds that a birth is unwanted or on the odds that a birth to an unmarried woman is mistimed. Among currently married women, those who are poorer are more likely than women above 150% of the poverty level to have a mistimed birth. Black women are more likely than either Hispanic or white women to report a birth as unwanted and are more likely than white women to say a wanted birth was mistimed.