ATLANTA (Reuters) - The number of test-tube babies born each year in the United States is underreported by states, a new federal study found on Wednesday, complicating efforts by researchers to understand the risks associated with pregnancies resulting from in vitro fertilizations.

The research by the National Center for Health Statistics compared birth certificate data in 27 states and Washington, D.C., with the numbers from a federal survey of the medical clinics that use assisted reproductive technology to join eggs and sperm in a laboratory dish.

There were 38,496 test-tube babies born in 2011, the federal survey found, which was more than twice as many as were reported on the states' birth certificates. The largest disparity between the numbers was in Florida and the lowest was in Utah, the study found.

Researchers do not know the reasons for the differences, said Marie Thoma, one of the study's authors. But improving the accuracy of the state data is key to better understanding the outcomes of these pregnancies, she said.

The number of children conceived using in vitro fertilization, sometimes called test-tube babies, has risen steadily in the United States since the early 1980s, the study said.

Due to the expense and complications associated with the procedure, multiple embryos are often implanted, leading to more births of twins and triplets, Thoma said. Such multiple births also carry higher risks.

A report issued earlier this year by the Society for Assisted Reproductive Technology found a record number of test-tube babies were born in 2012 in the United States.

The group, which represents more than 90 percent of the infertility clinics in the country, reported that in 2012 they performed 165,172 in vitro fertilization procedures, resulting in the birth of 61,740 babies.

In vitro technology accounted for just over 1.5 percent of the total U.S. births that year, more than ever before.