Social support, behavioral risk, and structural or demographic variables as well as acceptance of pregnancy were tested as determinants of late entry into prenatal care in a sample of 176 women in a rural county in California. The respondents were all those over age 18 served by four obstetric practices during a 4-month period ending in February 2000. One nurse-midwifery practice was included. Late entry into prenatal care during the first trimester occurred in 27.3% of the cases overall. Statistically significant independent variables in bivariate analyses were modeled in multivariate logistic regression. Stress, lack of family and friend support, Medicaid enrollment, age under 20 or over 34, low acceptance of pregnancy, and lack of a high school diploma were all predictors of late entry. Lack of family and friend support modified the effects of stress and Medicaid as payer. Although the determinants of late entry were remarkably complex in this sample, they have potential for public health intervention.