Expectant mothers who have gum disease are less likely to deliver their babies prematurely if they use mouthwash throughout their pregnancy, a new study suggests.

Pregnant women with gum disease, also called periodontal disease, are known to have more preemies than women with healthy gums.

But it's unclear whether that link is causal, and so whether better oral hygiene would make a difference.

The new study, although not ironclad proof, found that regularly using an alcohol-free mouth rinse appeared to cut women's risk of early labor by about three-quarters.

"I think this is extremely encouraging," said Dr. Steven Offenbacher, a professor at the University of North Carolina's School of Dentistry, who was not involved in this study.

"We haven't known the best way to manage these patients."

The research team, which included staff and funding from Procter and Gamble, the company that markets the mouthwash used in the study, asked 71 pregnant women with gum disease to rinse twice a day for 30 seconds with Crest Pro Health mouthwash. The mouthwash does not contain alcohol.

They compared the number of pre-term births among this group to 155 pregnant women who also had gum disease, but rinsed only with water.

Among the water-only group, 34 mothers -- or about one in five -- delivered their babies prematurely, before 35 weeks of pregnancy.

In the mouthwash group, just four mothers delivered their babies prematurely, which is about one in 20 births.

The difference between the groups "was just incredible," said Dr. Marjorie Jeffcoat, the lead author of the study and a professor at the University of Pennsylvania School of Dental Medicine.

But she pointed out that the women knew which treatment they were getting -- water or mouthwash -- which in principle might have influenced the results.

Jeffcoat's team did not identify why the mouthwash was linked to fewer premature babies, but gum disease might play a role.

Gum disease starts when bacteria on the teeth infect the gums and cause swelling, and pregnancy can exacerbate the condition.

Inflammation in gum disease involves the hormone-like substance prostaglandin E2, Jeffcoat explained. This same chemical is involved in labor.

Her hypothesis is that gum disease leads to inflammation and more prostaglandin E2 circulating through the body, which might then spark an early labor. On the flip side, by treating the gum disease, women can cut their prostaglandin E2 levels and reduce their risk of going into labor early, Jeffcoat believes.

The study, published in the American Journal of Obstetrics and Gynecology, found that mouthwash did appear to help the gum disease.

Mothers who didn't use the mouthwash had more inflammation and sites along the gums where the tissue would bleed.

Aggressive teeth-cleaning is also used to fight gum disease during pregnancy, and a study from April found the procedure appeared to be safe for expectant mothers.

But whether it has an effect on pre-term births is still unclear.

"We are seeing that those types of treatments are usually not adequate to prevent mothers from getting worse during pregnancies," Offenbacher told Reuters Health.

Mouthwash is a less-invasive alternative to teeth cleaning, and it typically costs less than $10 a bottle.

But, Jeffcoat cautioned, "Obviously, if you don't have periodontal disease this is not going to help" your risk of pre-term birth.

Both dentists agreed it's important for pregnant women to take care of their oral health.

"They need to use a soft toothbrush and floss the right way," wrapping the floss around the tooth, Jeffcoat told Reuters Health earlier this year. "The first goal with almost all dental disease is prevention, prevention, prevention."

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