An analysis of United States birth certificate records for the calendar year 1977 indicated that women who failed to seek prenatal care were at increased risk of delivery of a low-birth weight infant, even after social factors were considered. However, the strength of association, as well as the population impact, between antenatal care and birth weight varied by social group. Prenatal care had the greatest observed impact for socially disadvantaged women, because of their high overall risk of delivery of low-birth weight infants. From these observations, it appears that the efficacy of antenatal services is modified by social situation. As a result, summary evaluations of prenatal care impact may underestimate the true value of these services for certain social groups.