Population-based studies that include women who use both modern and traditional maternal health care are necessary to identify factors associated with the size of a baby at birth. This study examines the role of antenatal care on small size at birth based on the 2003 Nigeria Demographic and Health Survey data. The study finds that antenatal care as measured by tetanus toxoid injections and women who were provided guidance on where to go for pregnancy complications (a proxy for antenatal care) are associated with lower odds of giving birth to small-sized babies suggesting that the content of antenatal care is important in judging its quality and effect. Beyond antenatal care, a predominant factor associated with size of baby at birth is maternal nutritional status. Women with higher weight for height scores (>120) and taller mothers (>160 cm) had a lower incidence of small size at birth, whereas shorter maternal stature increases the risk of small birth sizes. The odds of small birth size are lower among women of high socioeconomic status (SES) in urban areas which is attributed to the expected positive correlation between SES and the utilization of antenatal care services. However, in rural areas, even poorer women had lower odds of the incidence of small birth size, as compared to poorest women implying that in addition to SES, antenatal care had a direct effect on size at birth in rural areas. Inter-regional comparisons also indicate that women in urban north-central and south-south parts of Nigeria were more likely to deliver small babies as compared to those in the south-west. But women in the north-west, living in rural areas, were less likely to deliver small babies compared to those in rural south-south. The findings suggest that selectively targeted interventions such as maternal education, maternal nutrition, routine tetanus toxoid injections and advocacy programmes aimed at mobilizing religious leaders as agents of sensitization and change may help in ensuring adequate care and better birth outcomes in their respective communities taking regional disparities into consideration.