Objectives To examine the temporal trend and risk factors associated with lack of prenatal care and the association between lack of prenatal care and adverse outcomes in pregnancies complicated by syphilis.

Methods We conducted a retrospective cohort study using US vital statistic data (2016–2022). We included non-anomalous, singleton live births delivered by pregnancies complicated by syphilis at 24–42 weeks of gestation in a hospital. The exposure variable was prenatal care (had care vs no care). We used Adequacy of Prenatal Care Utilization Index. Adverse outcomes were examined. The annual per cent change (APC) was calculated to assess the trend of lacking prenatal care. Multivariable Poisson regression models with robust error variance were used to examine the association. Adjusted relative risks (aRR) with 95% CIs were calculated.

Results Among the study population of 38 583 live births, 2417 (6.3%) lacked prenatal care. The rate of lacking prenatal care increased over time (APC=11.4; 95% CI 4.7 to 18.5). Maternal education, insurance, nulliparity, prepregnancy body mass index, prior preterm birth, smoking, diabetes, hepatitis C and year of delivery were associated with lacking prenatal care. Compared with individuals with prenatal care, those without care were associated with a higher risk of preterm birth <37 weeks (aRR 2.57; 95% CI 2.40 to 2.74) and <34 weeks (aRR 3.56; 95% CI 3.13 to 4.05). Compared with patients with adequate care, those without care were associated with a higher risk of preterm birth <37 weeks (aRR 5.07; 95% CI 4.59 to 5.60) and <34 weeks (aRR 6.42; 95% CI 5.32 to 7.75). Similar associations were shown in other adverse outcomes.

Conclusions Among pregnancies complicated by syphilis in the USA, there was an upward trend in lack of prenatal care from 2016 to 2022. Modifiable factors of lacking prenatal care were identified. The risks of adverse outcomes were higher among individuals without prenatal care.