San Juan, Puerto Rico ? The first thing Dr. Marta Valcarcel notices on a walk through a nursery full of tiny, heavily bandaged babies is the empty incubator in the corner.

She calls out to a nurse who only shakes her head. The infant who came in days earlier weighing less than 3 1/2 pounds had succumbed to illness — another child born too early to survive.

“I try to distance myself or I would cry all the time,” says Valcarcel, who runs the largest neonatal intensive care unit in this U.S. Caribbean territory. “We rejoice at the ones that survive.”

Her unit at a San Juan hospital is pivotal in the struggle to save a growing number of babies born well before their due dates. With nearly 20 percent of infants born before 37 weeks, Puerto Rico has the highest pre-term birth rate in the United States and one of the highest in the world.

Poverty in Puerto Rico, which has half the per capita income of the poorest U.S. state, can be a factor. Also, 48 percent of the island’s births are performed by Cesarean section, compared with 32 percent on the U.S. mainland. In some cases doctors opt to deliver babies by C-section before they reach term in hopes of avoiding complications.

But researchers say that’s not enough to explain why the island’s pre-term birth rate has jumped more than 50 percent over the last two decades.

The National Institutes of Health last month awarded $9.9 million for a team of university researchers to study whether contaminants in Puerto Rico’s groundwater are contributing to the problem.

The NIH-funded study will follow 900 pregnant women on the island’s north coast, collecting data on their drinking water and everyday activities to measure potential exposure to contaminants.

The densely populated island of 4 million people has a legacy of industrial contamination and several overflowing landfills along its northern coast, where the karst geology of soluble rock allows water to enter underground aquifers with little filtering.

“The only thing we are really sure of is we need to do more investigation,” said Alma Seda, executive director of Puerto Rico’s chapter of the March of Dimes.

Premature birth can lead to brain hemorrhaging, breathing problems and holes in the intestines. It is the No. 1 cause of infant mortality, and babies who survive can suffer from mental retardation or other lifelong handicaps, particularly the small minority born before 32 weeks’ gestation.

The March of Dimes has also linked a high prevalence of behavioral problems and learning disabilities in island schools to pre-term births.

Local experts say they welcome any theories because the known risk factors do not seem to explain the dramatic rise. More women are receiving prenatal care early in their pregnancies, fewer pregnant women are smoking, and there are fewer multiple births compared with the U.S. mainland, where fertility treatments are more popular.

Doctors who treat the babies say they suspect a combination of factors, including a degree of social deterioration. Edwin Soot Tapia, a neonatologist based in the north coast town of Arecibo, said he sees a growing number of premature infants born to teenagers who consume drugs and alcohol during their pregnancies.