OBJECTIVE To determine the seroprevalence of syphilis in pregnant women. METHODS A seroepidemiologic survey was conducted in 1857 women giving birth at a general hospital in the city of San Luis Potosi, Mexico. RESULTS Five women (0.27%) were diagnosed with syphilis at the time of delivery. Maternal factors associated with a greater likelihood of syphilis included older age, a higher number of pregnancies and living out of wedlock. CONCLUSIONS The number of new bhorns exposed to syphilis during pregnancy in San Luis Potosi is underestimated. The results of this study support the need to identify syphilis in infected mothers at the time of delivery.