THURSDAY, Jan. 29 (HealthDay News) -- Treating gum disease doesn't decrease a woman's risk of preterm birth, according to a U.S. study that challenges previous research suggesting a link.

As a result of those earlier findings, insurers and health-care providers started to recommend that pregnant women undergo dental "deep cleaning" (scaling and root planing) to reduce the risk of preterm delivery.

This new study, overseen by the University of North Carolina at Chapel Hill School of Dentistry, included 1,800 pregnant women with periodontal (gum) disease. The women were randomly assigned to two groups: One received periodontal treatment before 23 weeks gestation, while the other group did not receive treatment.

Overall, the two groups showed no significant differences in obstetric or neonatal outcomes. The findings were presented Thursday at the Society for Maternal-Fetal Medicine annual meeting, in San Diego.

"The biggest implication of this study is that this level of standard periodontal care will not affect the birth outcome," Dr. Amy Murtha, director of obstetrics research at Duke University Medical Center, said in an university news release. Duke was one of the medical schools participating in the study.

"That's not to say pregnant women should not get dental exams and treatment as needed; they should. Our study emphasizes that treating periodontal disease during pregnancy is safe, but that standard periodontal care is not enough," to reduce the risk of preterm birth, said Murtha, who presented the findings at the meeting.

More research is needed to better understand the relationship between gum disease and preterm birth.

"Periodontal disease and poor pregnancy outcomes travel together, but we don't know why," Murtha said. She suggested that preterm birth and gum disease may share a common underlying trait, such as an exaggerated inflammatory response.

It's also unclear why pregnancy appears to be associated with the onset and progression (worsening) of gum disease. Progression of periodontal disease occurs in about 25 percent of pregnancies.