(120) Infant Birthweight Does Not Differ Following Exposure to Maternal Syphilis After 20 Weeks Gestation

Emerging data suggest fetal growth may be impacted by exposure to syphilis after 20 weeks of gestation. We compared birthweight among infants of gravidas with syphilis diagnosed after 20 weeks and unexposed infants.

Study Design:

This is a retrospective matched cohort study of gravidas with syphilis diagnosed after 20 weeks’ gestation at a large public hospital. We excluded patients with substance use and syphilis treatment prior to pregnancy, unless early syphilis was diagnosed. Cases were matched to controls (1:2) without syphilis based on maternal age ±5y, race, ethnicity, gestational and pregestational diabetes, chronic or gestational hypertension, and preeclampsia. Comparisons were made using mixed effects models to account for matching. 

Results:

From January 2010 through June 2025, 173 cases of maternal syphilis were diagnosed at a median gestational age of 29 weeks (IQR 23-33) and matched to 346 uninfected controls. Among those with syphilis, 77 (45%) had early-stage syphilis (primary, secondary, or early latent) and 96 (55%) latent, late or unknown duration. Of 144(83%) who received intramuscular benzathine penicillin G before delivery, the median gestational age at first dose was 29 weeks (IQR 24.3-32.4). Median gestational age at birth was 39 weeks for both groups (IQR 38-40, p=0.72). Median infant birthweight (g) did not differ between syphilis exposed and control infants (3200 [IQR 2778-3486] vs 3248 [IQR 2912-3600], mean difference -60.5g [95%CI,-175.0, 54.0]). Infants in the syphilis cohort had lower odds of small for gestational age (SGA) diagnosis compared to controls (13[8%] vs. 50[14%], respectively; OR 0.47 [95%CI, 0.24-0.89]). Stillbirth occurred in 4(2%) with syphilis compared to 1(0.3%) in controls (p=0.06). 

Conclusion:

Birthweight did not differ among infants exposed to maternal syphilis diagnosed after 20 weeks’ gestation compared to unexposed infants after matching for maternal medical comorbidities and excluding substance misuse.