Objectives To investigate the risk factors for infection and the outcome of pregnancy in women diagnosed with syphilis using a molecular approach.

Methods Case-control study of pregnant women admitted to a maternity ward, with the cases classified as early or latent maternal syphilis, based on clinical-serological diagnoses. The DNA of total peripheral blood was used to detect the polA gene using nested PCR (nPCR). The case and control groups were divided into subgroups based on whether the birth was successful (infant survived) or had a lethal outcome (miscarriage, stillbirth or neonatal death).

Results The frequency of maternal syphilis was 1% (237/25 600), considering both those that had live births (71.3%, 169/237) and those with a lethal outcome (28.7%, 68/237), with a higher detection rate being provided by the nPCR in women with early syphilis. The cases of nPCR(+) congenital syphilis were more frequent in the women with early syphilis, nPCR(+) mothers and those who did not have treatment during the prenatal. The risk of maternal syphilis was greater in women who had not received preventive counselling, initiated sexual activity at 16 years of age or younger, had multiple partners, used drugs, were from households with a low income and poor sanitation, and had a history of miscarriage.

Conclusions The risk factors for congenital syphilis are closely related to the health of the mother, reflecting the lack of adequate prenatal care. The high frequency of maternal syphilis was associated in particular to the socio-economic conditions of the mother and her sexual and reproductive health.