Many moms-to-be opt for blood tests during pregnancy to check for fetal disorders such as Down syndrome. In rare instances, these tests can reveal something unexpected — hints of a hidden cancer in the woman.

In a study of 107 pregnant women whose test results were unusual, 52 were ultimately diagnosed with cancer. Most of them were treated and are now in remission, although seven with advanced cancers died.

“They looked like healthy, young women and they reported themselves as being healthy,” said Dr. Diana Bianchi, the senior author of the government study recently published in the New England Journal of Medicine.

Of the discovered cancers, lymphoma blood cancers were the most common, followed by colon and breast cancers.

The blood test is called cell-free DNA sequencing. It looks for fetal problems in DNA fragments shed from the placenta into the mother’s bloodstream. It also can pick up DNA fragments shed by cancer cells.

Of the millions of pregnant women each year who have a cell-free DNA test, 1 in 10,000 will get a result back that is unusual and difficult to interpret, neither positive or negative for a fetal abnormality. This small number of people — perhaps only 250 a year in the U.S. — may be at risk for cancer.

“They and their care providers need to take the results seriously and have additional testing because in that population there is a 48% risk of cancer,” said Bianchi, who leads the National Institute of Child Health and Human Development.

The researchers concluded that it’s best to do a whole-body MRI to look for cancer. A physical exam or taking a family history is not enough, Bianchi said.

The research identified a recognizable “very chaotic” pattern in the DNA sequencing of the women diagnosed with cancer, Bianchi said.