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Sexually transmitted infections (STIs) during pregnancy were associated with a higher risk of preterm birth, researchers found in a retrospective study.

In a nationwide analysis of birth certificate data involving over 14 million mothers, a diagnosis of syphilis, gonorrhea, or chlamydia was tied to a higher risk of preterm birth (adjusted OR 1.06, 95% CI 1.05-1.07), reported Wei Bao, MD, PhD, of the University of Iowa in Iowa City, and colleagues.

Risk of preterm birth was highest for syphilis (aOR 1.17, 95% CI 1.11-1.22), followed by gonorrhea (aOR 1.11, 95% CI 1.08-1.15) and chlamydia (aOR 1.03, 95% CI 1.02-1.04), the authors wrote in .

About 10% of all births are preterm, which are increasing in prevalence, the authors noted. However, studies have been limited and inconclusive in providing evidence linking STIs to preterm birth.

Bao and colleagues examined birth certificate data from the National Vital Statistics System on 14,373,023 mothers from 2016 to 2019. Mothers were included if they had a single live birth and documented gestational age. The primary outcome assessed preterm births, evidenced by births prior to 37 weeks.

Women in the study had an average age of about 29, and about half were non-Hispanic white (51%), while 24% were Hispanic. Overall, 3.3% of women had a prior preterm birth, 58.3% did not have a prior preterm birth, 38% were having their first child, and data were missing for the rest.

About 2% of women had chlamydia, 0.3% had gonorrhea, and 0.1% had syphilis. Higher rates of all three were seen among women younger than 25, non-Hispanic Black women, women with less than a high school education, women who smoked during pregnancy, unmarried women, and those on Medicaid.

Preterm births accounted for 8% of all births, but the rate of preterm birth was 13% for women with syphilis, 12% for those with gonorrhea, and 10% for those with chlamydia, the authors said.

In , Emily Adhikari, MD, and Scott Roberts, MD, MS, both of the University of Texas Southwestern Medical Center in Dallas, reiterated a primary limitation of the study: mainly that it was entirely based on observational data.

"With only observational studies to interpret, we are faced with a mountain of sometimes conflicting data," Adhikari and Roberts said. "While the massive sample size of this study and point estimates with tight confidence intervals might suggest precision, they do not necessarily suggest validity."

In fact, they noted that they were "not aware of any randomized controlled trials evaluating the effect of treatment or lack of treatment of chlamydia, gonorrhea, or syphilis infections on primary prevention of preterm birth," they wrote, though they added that exposure or interventional trials involving STIs would be "difficult to justify and lack beneficence."

The analysis had several other limitations, the researchers acknowledged, including the lack of detailed information about STI treatment, the lack of preterm birth subtypes, and potential for residual confounding.

"We are still faced with the challenge of understanding the nature of this association: to what degree do STIs cause preterm birth, and how effective can STI treatment be in reducing preterm birth in the U.S.?" the editorialists wrote. "Without a better understanding of which infections were and were not treated, we have little to guide a search for targeted interventions to prevent preterm birth in a cohort with treatable infectious diseases."

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