OBJECTIVES: This study examined the relationship between compliance with the US Public Health Service guidelines for prenatal care and the rate of prenatal hospitalization. METHODS: For all women admitted to a Boston referral center during January and February 1993 with a pregnancy of at least 18 weeks gestation (n = 1400), a proportional hazards model was used to examine factors associated with prenatal hospitalization. RESULTS: Prenatal hospitalization occurred during 248 (17.7%) pregnancies. The median length of stay for all prenatal admissions was 4 days; the medial total charge was $5667. Prior medical and obstetrical problems were strongly associated with prenatal hospitalization. After adjustment for age, race, and medical and obstetrical complications, women who received less than 70% of the prenatal care recommended were significantly more likely to be hospitalized (relative risk [RR] = 2.14, 95% confidence interval [CI] 1.50, 3.06). CONCLUSIONS: Prenatal hospitalization is a common, costly complication of pregnancy. Because of its association with compliance with the Public Health Service guidelines for the content of prenatal care, prenatal hospitalization may be a sentinel indicator of inadequate prenatal care amenable to intervention.