Association of Psychoactive Substance Use With Syphilis and HIV Positivity Among Pregnant Women Receiving Prenatal Care in Belém, Northern Brazil.

Vertical transmission of sexually transmitted infections remains a public health concern, particularly in socially vulnerable settings. Although Brazil has achieved the elimination of mother-to-child transmission of HIV, syphilis during pregnancy continues to pose significant challenges, especially in the northern region. This cross-sectional study aimed to estimate the prevalence of HIV and syphilis and identify associated factors among pregnant women receiving prenatal care in Belém, Pará, Brazil. Between April and June 2021, 398 pregnant women were interviewed using a structured clinical-epidemiological questionnaire and underwent laboratory testing. HIV diagnosis was established using fourth-generation serological assays with immunoblot confirmation, and syphilis diagnosis was based on nontreponemal testing followed by treponemal confirmation. Descriptive analyses were performed, followed by bivariate analyses using Fisher's exact test, chi-square test with Monte Carlo simulation, and Mann-Whitney U test, and Poisson regression with robust variance was applied to estimate prevalence ratios for syphilis. The prevalence of syphilis was 6.36%, and HIV prevalence was 1.01%. Psychoactive substance use was the only variable independently associated with syphilis positivity, while other sociodemographic and behavioral factors showed descriptive trends without statistical significance, and no significant associations were observed for HIV infection. These findings indicate that persistent social and behavioral vulnerabilities, particularly psychoactive substance use, contribute to ongoing syphilis transmission during pregnancy, underscoring the importance of integrating substance use screening, targeted counseling, and harm-reduction strategies into prenatal care to advance efforts toward eliminating vertical transmission of sexually transmitted infections.