This study identified a high estimated prevalence of GS. Latin American and the Caribbean had the third highest estimated prevalence of GS in the world (0.42%), behind Africa (1.68%) and the Eastern Mediterranean (0.57%). ^{( 1 , 6 )} In Brazil, the estimated prevalence of GS ranges from 1.02% to 28.60%, depending on the source. ^{( 8 , 15 - 18 )} These variations can be partially explained by differences in the scope of the study, definition adopted for syphilis infection, data source, and inequalities in age distribution. ^{( 8 , 17 )}

The variables that represent the behavioral characteristics and history of STIs were significantly associated with GS at all levels of analysis, highlighting the importance of these factors. It is possible that the increase in syphilis is explained by vulnerable situations that involve behavioral characteristics of unsafe sexual practices. ^{( 10 , 16 , 19 )} However, although they did not all remain in the final model, it is important to discuss the factors associated with GS in each block. Poverty results in vulnerable conditions, such as behavioral or reproductive, which may be associated with a decrease in access to and inadequacy of prenatal care. ^{( 10 , 16 )} We observed a higher prevalence of GS in women who started prenatal care later and had a lower frequency of appointments, demonstrating a close relationship between syphilis and prenatal care. ^{( 8 , 10 , 15 , 16 )}

Early sexual initiation, as in other studies, may be related to a pattern of risky sexual behavior for STIs. ^{( 10 , 16 )} The fact that most women report never using condoms or using them in less than half of their sexual intercourse encounters during pregnancy reveals that a large proportion of women are at risk of contracting STIs. ^{( 10 , 16 , 19 )}

In this study, some variables were investigated with only the women, and there was no interview with their sexual partners. Having a partner with a history of syphilis and/or illicit drug use was associated with the occurrence of syphilis in women. The incidence of CS can be reduced by using relatively simple interventions. However, as long as syphilis is prevalent among adults, the potential for mother-to-child transmission remains high. Thus, the sustainable elimination of CS requires coordinated efforts to simultaneously reduce the rate of acquired syphilis in adults. ^{( 20 )} In this sense, prenatal care provides an ideal opportunity to approach women about their sexual partnerships.

Contact tracing of the sexual partners of women with syphilis is an opportunity for the detection of syphilis in men, treatment, and reduction of the transmission chain. ^{( 3 , 21 , 22 )} However, it is known that contacting a partner is not easy if he is not participating in prenatal care. ^{( 16 , 22 )} We emphasize the need to implement the National Policy for Integral Attention to Men’s Health (PNAISH - Política Nacional de Atenção Integral à Saúde dos Homens ), which, through the partner’s prenatal care, points to an opportunity to connect this group to health units and carry out preventive actions beyond the control of syphilis. ^{( 22 , 23 )}

The use of illicit drugs by partners is a vulnerability factor for GS, as found in other studies. ^{( 10 , 18 , 24 )} Risk behaviors for STIs associated with the use of illicit drugs include exchanging sex for drugs, not using condoms, a greater number of sexual partners, and unplanned sexual activity, among others. ^{( 10 , 19 )}

Syphilis surveillance in the United States from 2013 to 2017 showed that the proportion of primary and secondary syphilis cases that reported injectable drug, methamphetamine, or heroin use (or that reported sex with a person who uses injectable drugs) in the past 12 months was more than double among women and among men who have sex with women only. ^{( 24 )}

The remarkable increase in the number of primary and secondary syphilis cases among women of childbearing age is mirrored by the increasing number of CS cases and increasing infant mortality. ^{( 4 , 24 )} All stages of syphilis in pregnant women pose a risk of transmission to the fetus, but the risk is considerably higher in the early stages than in the later stages of syphilis. These data suggest a link between illicit drug use and the rise in CS in the United States. ^{( 4 , 24 )} Another study showed the temporal and individual relationship of the associations of three epidemics: the use of smoked cocaine (“freebase” and crack), STI associated with genital ulcers, and heterosexually transmitted HIV infection. ^{( 25 )}

Similar to other studies, a greater number of sexual partners during pregnancy and history of syphilis or other STIs in the pregnant woman were both associated with syphilis in the current pregnancy. ^{( 10 , 16 - 18 )} The adoption of safe sexual behaviors does not depend solely on the education level, income, knowledge of safe sexual behaviors, and access to condoms, but also on the meanings attributed to sexuality and the care for one’s own health. ^{( 10 )} The actions proposed to mitigate syphilis among women still face the challenge of connecting prevention and care, and promoting sexual and reproductive autonomy. ^{( 10 , 16 )}

Alcohol consumption during pregnancy is associated with GS, as has been reported in other studies. ^{( 17 , 18 )} Alcohol consumption increases the chance of risky behaviors, such as not using condoms and increasing the number of sexual partners. ^{( 19 )}

Similar to other studies, low education and income were associated with GS. ^{( 8 - 10 , 16 , 26 )} The proportion of people affected by syphilis remains high among the poorest and most marginalized populations in the Americas, and Brazil has historically been among the most unequal countries in the region. ^{( 1 )} In Brazil, there is a direct association between education and income inequality in which salary differentials are associated with schooling. ^{( 27 )} Years of study can influence the way health problems are perceived, and interfere with the adequacy of prenatal care and syphilis treatment. ^{( 27 )}

The association of syphilis with the report of a woman not having a partner has been found in other studies. ^{( 26 )} Socio-family insufficiency is one of the main health determinants of pregnant women. ^{( 28 )} Some authors suggest that having a partner is an enabling factor in the use of prenatal services. ^{( 15 , 28 )}

Other studies have shown that women with GS have an increased risk of stillbirth. ^{( 29 , 30 )} Stillbirth history is important for suspected syphilis infections. ^{( 3 , 29 , 30 )}

The association between receiving prenatal care from FHS teams and GS suggests problems in the functioning of these teams and/or can be attributed to the worse sociodemographic conditions of women residing in places where this strategy was primarily implemented in the municipalit chosen by the criterion of poverty ^{( 9 , 31 )} and mapped by the Institute of Applied Economic Research (IPEA) Hunger Map. ^{( 31 )} This map indicated that there were 17,000 families below the poverty line in Juiz de Fora. ^{( 31 )}

As in other studies, our results indicate that women who start prenatal care later and have a lower frequency of appointments are at greater risk for the occurrence of GS and, consequently, its adverse effects. ^{( 15 , 25 )} Having performed “two or more tests for syphilis” was associated with both “beginning prenatal care in the first trimester” (79.8%, p<0.001) and having “six or more consultations” (89.0%, p<0.001) (data not shown in tables). This finding reveals that early initiation of prenatal care, continuity of care, and procedures for syphilis detection are an important part of the effectiveness of prenatal care. ^{( 9 , 10 , 15 )}

An FHS team can minimize the effects of socioeconomic inequalities. ^{( 9 , 12 )} In some municipalities, in the basic network, it has already been demonstrated that pregnant women visited by community health workers start prenatal care earlier and have more prenatal appointments, laboratory tests, and counseling. ^{( 9 )} In addition, the implementation of rapid tests for syphilis in the PHC units, counseling, and immediate administration of penicillin would allow for the reduction of syphilis in the adult population, CS, and its more severe forms. ^{( 1 , 32 )}

Figueiredo et al. carried out an ecological study using part of the data from the Brazilian Access and Quality Improvement Program in Primary Care (PMAQ-AB - Programa de Melhoria do Acesso e da Qualidade na Atenção Básica) assessment conducted in 2014. Their objective was to analyze the relationship between the approaches of diagnosis and treatment of syphilis in primary care and the incidence of GS and CS. Municipalities with reduced vertical transmission had higher median percentages of teams offering rapid tests (increased ability to detect GS) and patients taking penicillin, demonstrating a relationship between these actions and the reduction of CS. ^{( 32 )}

These results indicate that syphilis is a health and social problem that is directly related to poverty, low education, and vulnerable lifestyles. ^{( 1 , 5 , 7 - 10 , 16 )} Vulnerable women should be offered specific prenatal care strategies that include sexual partnerships, structured actions for prevention, and assistance with active search, healthcare network support, and social services.

Limitations of this study include the possibility of gaps in information about tests for syphilis performed during prenatal care, especially for women without a diagnosis of syphilis, and omission or issuance of untrue information in responses regarding behavior. However, we reinforced the validity of the results by analyzing various data sources, correcting data collection instruments used in the pre-test and pilot study, and providing a welcoming environment at the time of the interview.