Thirty-six years after opening, the Birth Center in Bryn Mawr is booming and just celebrated a milestone - its 10,000th baby.

In comparison, Pennsylvania Hospital, which has Philadelphia's largest maternity unit, delivers 10,000 babies every two years.

Therein lies one of the biggest ironies of maternity care in the United States.

Demand for midwife-led birthing centers - with their nurturing, low-tech, cost-saving approach to pregnancy and childbirth - has been growing like, well, a newborn. The number of centers jumped from 195 four years ago to 255 now, a 30 percent increase, according to the American Association of Birth Centers.

Yet, out-of-hospital deliveries remain rare, and most happen in homes, not birthing centers. Fewer than 17,000 babies were born in birthing centers in 2013, or 0.4 percent of the country's four million births, federal data show.

"Each year, there has been more and more demand," said Kathryn Boockvar, executive director of the Bryn Mawr center. "But there aren't enough of us. There are only five licensed birth centers in Pennsylvania."

Actually, five is a comparative bounty; 1.2 percent of Pennsylvania births were in birthing centers in 2013 - three times the national average.

Why aren't birth centers a bigger part of maternity care?

Women themselves have conflicting attitudes, as Boston University public health professor Eugene Declercq has found in periodic national surveys of mothers who gave birth in hospitals. The majority (74 percent) agreed with the statement "Newer maternity tests and treatments are generally improvements over older ones." Yet, 39 percent said they would consider a birth center if they had another pregnancy, and 25 percent definitely wanted that nonhospital option.

"What's happening in a hospital birth that makes these women not want to do it again?" Declercq asked.

The question was rhetorical. As founder of Birth by the Numbers, a website on practices and outcomes, Declercq has tracked the U.S. reliance on invasive technology, including cesarean section, labor induction, and fetal monitoring.

Another reason birth centers remain on the fringes involves regulatory and financial barriers.

"Consider who loses money if birthing centers become popular," Declercq wrote in a recent opinion piece. "Large hospitals, pharmaceutical companies, obstetricians, and anesthesiologists - powerful groups who believe deeply that the use of their products and services is not only a source of income, but the safest option for mothers and babies."

The thing is, birthing centers strive not to compromise safety. They turn away pregnant women with complicating factors such as high blood pressure, obesity, and carrying twins. About 1 in 8 women who start labor in a birthing center wind up being transferred to a hospital.

"We have clinical practice guidelines. We can only care for lower-risk patients," said Boockvar. "I wish I could say every woman could come to the Birth Center, but she can't."

Frances and Matthew Brodsky of Elkins Park on Dec. 4 welcomed their third child, Vera, as the center's 10,000th baby. Their two older children were also born there.

"I wanted something natural, not in a hospital, but not at home," said Frances. "I was also fortunate to have the same midwife for all three of my children. It's a wonderful place."

mmccullough@phillynews.com

215-854-2720 @repopter