Background and aims of the study: Congenital syphilis is a rare complication of syphilis in pregnant women. Vertical transmission may occur at any time during pregnancy. The incidence of congenital syphilis has been increasing worldwide. Congenital syphilis has been a notifiable disease for many years in Switzerland but reporting does not include maternal features associated with syphilis in pregnancy or infant's subsequent development. We described syphilis cases among pregnant women screened over a 10-year period in Switzerland and subsequent cases of congenital syphilis, in order to identify maternal risk profiles and to optimise prevention. Second, we compared the characteristics of pregnant women screened early (1st trimester) vs late in pregnancy (2nd or 3rd trimester). Finally, we assessed the risk factors for premature birth among these women with syphilis.

Methods: A multicentre retrospective study conducted in Swiss hospitals from 2012 to 2021, including pregnant women who screened positive for syphilis (Treponema pallidum haemagglutination assay [TPHA] / T. pallidum particle agglutination assay [TPPA ] ≥1:80) and newborns exposed to T. pallidum in utero and/or congenitally infected and with a positive syphilis serology at birth. Data were collected from medical records.

Results: A total of 147 syphilis-positive pregnant women and 102 infants were included. A history of treated syphilis was known for 44% (65/147) of the mothers corresponding to a serological scar and the remaining 56% (82/147) were newly identified syphilis cases. Syphilis screening was done during the first trimester in 54%, second trimester in 29% and third trimester in 13% of cases. Two babies were diagnosed with congenital syphilis (1.96%). Several potential factors that could contribute to women's risk of syphilis during pregnancy were identified such as a foreign origin (93% of mothers), lack of healthcare insurance (25%), no employment status (37%), drug use (5%), co-infection with other sexually transmitted infections (24%) and a late first antenatal consultation (42%). The number of pregnant women without insurance was higher in women diagnosed in the second or third trimester than in those diagnosed in the first trimester (odds ratio 0.41; 95% CI 0.19-0.89; p = 0.024). Syphilis diagnosed in the second or third trimester was associated with a late first antenatal consultation (odds ratio 77.82; 95% CI 9.81-617.21; p <0.001). A high rate of intrauterine growth retardation and of preterm birth was observed in newborns (18% versus 6% in Switzerland in 2022).

Conclusion: Congenital syphilis remains rare in Switzerland. However, we found potential maternal factors associated with a positive syphilis serology during pregnancy, which can help to improve future prevention measures. The study protocol was registered on ClinicalTrials.gov (ID NCT05975502).