STI screening linked to fewer preterm births

by Perelman School of Medicine at the University of Pennsylvania

edited by Gaby Clark, reviewed by Andrew Zinin

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Screening and treating pregnant patients for three common sexually transmitted infections (STIs) twice during pregnancy was associated with a 23% lower risk of preterm birth compared with standard care, according to a randomized clinical trial published in Clinical Infectious Diseases from researchers at the University of Pennsylvania and University of Southern California.

In practical terms, 14.5% of participants who received screening and treatment twice during pregnancy had a preterm birth, compared with 18.8% of those receiving standard care. Screening only once was not associated with a reduction in preterm birth.

"Complications from preterm birth are among the leading causes of death among children under 5 years of age globally. This study suggests that testing for STIs during pregnancy could help us find and treat infections that might otherwise go unnoticed, and that screening more than once may be important for reducing the risk of preterm birth," said co-lead author Andrew Medina-Marino, Ph.D., adjunct associate professor of psychiatry at the University of Pennsylvania Perelman School of Medicine.

"Because most of the infections we found caused no symptoms, waiting for symptoms alone means we may miss an opportunity to intervene."

Most infections caused no symptoms

The trial focused on three common, treatable STIs: chlamydia, gonorrhea and trichomoniasis. These infections have been linked to preterm birth and low birth weight. However, whether routine screening and treatment during pregnancy can improve birth outcomes has remained uncertain.

Researchers enrolled 2,247 pregnant participants at four government health clinics in South Africa and randomly assigned them to one of three groups: screening and treatment at their first prenatal visit with a test-of-cure three weeks later for women treated for a detected STI; screening and treatment at their first visit and again between 30 and 34 weeks of pregnancy; or standard care, in which treatment was based on symptoms.

At the start of the study, about 27% of participants tested positive for at least one of the three STIs, and approximately 80% of those infections caused no symptoms.

Among participants who were screened again in their third trimester, 9.2% tested positive at that visit. Some infections persisted or returned, while others were newly detected in participants who had tested negative earlier in pregnancy.

Results point to potential value of screening

Researchers found preterm birth among 14.5% of participants screened twice, compared with 18.8% receiving standard care. The one-time screening group had a nearly identical preterm birth rate to the standard-care group, at 18.9%.

The twice-during-pregnancy approach did not, however, significantly reduce the study's primary combined outcome of preterm birth or low birth weight. It also did not reduce low birth weight when researchers considered that outcome separately. The authors therefore caution that the finding on preterm birth needs further study.

"The results give us an important signal, but they don't settle the question of how often pregnant patients should be screened," Medina-Marino said. "The next step is understanding when screening can have the greatest impact and which patients are most likely to benefit."

Building evidence for prenatal screening

The optimal timing of STI screening during pregnancy remains unclear. In this trial, participants entered the study at a median of 13 weeks of pregnancy. Researchers said testing later in pregnancy may have provided another opportunity to identify persistent or newly acquired infections, although additional studies are needed to confirm that explanation.

The authors noted several limitations. The trial was conducted in a single South African setting where STI prevalence was high, which could limit how broadly the findings apply elsewhere. The study also was not designed to determine definitively whether screening benefited particular subgroups of patients.

Still, the researchers said the findings add to growing evidence about whether, when and for whom STI screening during pregnancy could improve birth outcomes, including in higher-income countries such as the United States.

Publication details

Andrew Medina-Marino et al., Antenatal Screening for Sexually Transmitted Infections to Reduce Preterm Birth or Low Birthweight (Philani Ndiphile Study): A Randomized 3-Group Trial, Clinical Infectious Diseases (2026). DOI: 10.1093/cid/ciag577

Journal information: Clinical Infectious Diseases

Key medical concepts

Premature BirthChlamydia InfectionsGonorrheaTrichomoniasis

Clinical categories

Obstetrics & gynecologyPregnancyWomen's healthInfectious diseasesCommon illnesses & Prevention Provided by Perelman School of Medicine at the University of Pennsylvania Who's behind this story?

Gaby Clark

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