Despite recent advances in the prevention of transmission of human immunodeficiency virus (HIV) infection from mother to child during pregnancy, infants continue to be born and infected with HIV, particularly in Africa. This study was undertaken to determine the seroprevalence of HIV infection among unbooked pregnant women in the Niger Delta of Nigeria. One hundred and eighteen consecutively recruited unbooked subjects presenting to the isolation ward at the University of Port Harcourt Teaching Hospital were screened for HIV. Among the 118 subjects studied, 30 (25.4%) were positive for HIV. HIV-1 was the predominant viral strain. Gestational age of subjects at presentation was 28–40 weeks and mean age was 35.04 ± 8.06 years. The majority of subjects were primigravidas 66 (55.9%), while 52 (44.1%) were multigravidas. The prevalence of HIV was significantly higher among unbooked pregnant women with less formal education: 14 (11.9%) compared with 9 (7.6%), 5 (4.2%), and 2 (1.7%) for those with primary, secondary, and tertiary education, respectively (P = 0.01). Among the occupational groups, the prevalence of HIV was significantly higher among traders 14 (11.9%) than in career women 5 (4.2%, P = 0.04). Multigravid women were more susceptible to HIV infection 17 (14.4%) than primigravid women. Perinatal mortality and emergency cesarean section was high among unbooked pregnant women. The prevalence of HIV observed amongst unbooked antenatal subjects in this study is significantly higher than those of booked patients in previous studies. These findings are very pertinent to health care delivery, because this pool of unbooked patients may not be benefiting from the Prevention of Maternal to Child Transmission program, thus increasing the pediatric HIV burden in our environment