New mother Kelly Barnes is heartbroken and angry.

She's heartbroken because she has endured what no mother should have to endure. While pregnant with twins, she lost one of them at 30 weeks. The other baby, Kinsleigh, was born with serious heart problems.

But Barnes is angry because her insurance company, Aetna, held up paying thousands of dollars in medical charges. The reason? The insurance company said the newborn might have been suffering from a pre-existing condition.

"I don't know how something could be pre-existing in a baby, so it was very shocking to me to see something like that," Barnes told "Good Morning America." "It's a slap in the face. Her medical bills are the last thing I should be worrying about. I should know that my baby is being taken care of."

"Under Aetna's own definition, in order to deny for pre-existing condition, there has to be medical advice or care that was rendered or given," Barnes' attorney, Tom Caldwell, said. "And in this case, of course, that would be real hard, given the fact the baby was still in the womb."

Barnes said she called Aetna hoping for a resolution.

"It's like you're talking to somebody who is reading from a script," Barnes said. "They don't have answers for you based on what you're telling them."

Finally, Barnes contacted "GMA." We called the insurance company in September and they claim that the pre-existing condition hold up was a simple coding error and have been paying back Barnes' claims since July. But Barnes said that no one ever told her that and it wasn't until "GMA" got involved that all the costs were finally paid this week.

"It is my personal belief that they will -- they do this to you, expecting you not to follow up with it," she said. "And I'm sure most people don't."

Consumer expert Deann Friedholm said what Barnes experienced is not uncommon.

"It really might be a clerical error, but let's face it, it is in the company's interest to not pay bills. The fewer bills they pay, the better their bottom line is," Friedholm said.

Kinsleigh still needs heart operations, but those Aetna said they'll be paying for. Even so, Barnes said she'll never forget the nightmare that Aetna put her and her family through.

Following the "GMA" investigation, Aetna apologized to Barnes.

CLICK HERE to read Aetna's response to our story.

Tips for the Insured

The following tips were provided by Deborah Senn, former Insurance Commissioner for the State of Washington and author of an upcoming book called "Survival Insurance."

Look at your Explanation of Benefits form very carefully.

Question everything and if it doesn't look right, ask.

If you have to challenge the companies, don't stop fighting.

When you talk to the insurance company, try to do it online. That way, you can get a record of the conversation online. This can be really helpful.