SEATTLE -- It's supposed to be the healthiest thing a mother can give her baby.

Now, a disturbing new study uncovers a troubling chemical in mothers' breast milk.

MORE ON THIS STORY More from the Environmental Working Group

Today, all new moms are told that breastfeeding passes many benefits on to their babies. But here's something moms probably don't count on: New research found all the women in a nationwide test had a toxic chemical in their breast milk.

That's prompting many to call for a ban on the chemical.

Valerie Vance decided to breastfeed before her baby Henrietta was born.

"I did. You know, you heard over and over -- that it's the best thing for the baby," she said.

Now new research raises concerns about mother's milk.

Chemicals used to prevent fires and known to cause developmental problems in animals have been found in the breast milk of American women. U.S. women had the highest levels in the world.

Researchers tested breast milk from just 20 women in 14 states, including Washington. The results mimic what larger studies have uncovered.

Fire retardants known as P.B.D.s are found in all kinds of everyday products from cell phones and hair dryers to coffee makers, paint and furniture.

Polyurethane foam -- the kind you find in mattresses, for instance -- is another source.

One theory is that the chemicals get into our bodies through hand to mouth contact, but how harmful are they to humans? There's no research to rely on.

"The bottom line is that we don't know what the effects of chronic low dose exposure like these are either on the mother or the child," said Dr. Tim Takaro, an expert in environmental medicine.

Dr. Takaro says breast milk is still one of the best things women can give their babies.

"Breast milk has so many beneficial attributes particularly for the immune system of a developing fetus," he said.

Valerie Vance says she'll definitely continue to breast feed Henrietta.

"It's the best part of my day. It just feels right," Vance said.

One surprise in the women tested: Women in the same town, presumably doing many of the same things had widely different levels of the fire retardant chemical in their breast milk. Prompting the question: what makes some women more susceptible?