Two rounds of prenatal STI screening lower preterm birth risk

Screening and treating pregnant patients for three common sexually transmitted infections (STIs) twice during pregnancy was associated with a 23 percent 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 percent of participants who received screening and treatment twice during pregnancy had a preterm birth, compared with 18.8 percent 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 five 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. Because most of the infections we found caused no symptoms, waiting for symptoms alone means we may miss an opportunity to intervene."

Andrew Medina-Marino, PhD, co-lead author, adjunct associate professor of Psychiatry, University of Pennsylvania Perelman School of Medicine

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 percent of participants tested positive for at least one of the three STIs, and approximately 80 percent of those infections caused no symptoms.

Among participants who were screened again in their third trimester, 9.2 percent 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 percent of participants screened twice, compared with 18.8 percent receiving standard care. The one-time screening group had a nearly identical preterm birth rate to the standard-care group, at 18.9 percent.

The twice-during-pregnancy approach did not, however, significantly reduce the study's primary combined outcome of preterm birth or low birthweight. It also did not reduce low birthweight 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.