BOSTON — Researchers say premature babies are at higher risk of complications, illness and even death. However, a natural hormone may keep expectant mothers from delivering too quickly.

Silvia Harrington's baby is not due until April, but she's restricted to bedrest. Recently, she went into premature labor.

"It makes you feel very unsafe because of the uncertainty of the situation," she said. "It creates a lot of scary moments."

Researchers said the number of premature babies born in the United States continues to grow -- 500,000 births every year. They said somewhere between 7 percent to 11 percent of deliveries in the United States are premature births.

Dr. Lucy Bayer-Zwirillo is now looking for an answer to the problem. The researcher is trying to find out if a hormone called progesterone can keep babies from being born too soon.

"Since the '70s, actually, there was a thought that progesterone might be a good drug to prevent prematurity," she said.

However, it wasn't until recently that investigators began testing it. Scientists believe the natural hormone works by relaxing muscles in the uterus, thereby preventing premature contractions.

So far, the results have been positive. One small study found the hormone worked.

"It had a tenfold decrease in premature delivery," said Bayer-Zwirillo.

Now, there's a large, nationwide

clinical trial

to see just how effective progesterone is. The hormone is in a gel and expectant moms in the study are being asked to apply it daily.