Aug. 15, 2006  -- Lydia Fairchild was a proud mother who faced the most unusual of challenges. She had to fight in court to prove the children born from her body were her own.

"I knew that I carried them, and I knew that I delivered them. There was no doubt in my mind," Fairchild said.

Fairchild's fight for her kids began when she was 26-years-old, unemployed and applying for public assistance in Washington state. Everyone in her family had to be tested to prove they were all related.

The Department of Social Services called Fairchild and told her to come in immediately. What Fairchild thought was a routine meeting with a social worker turned into an interrogation. The proud mother was suddenly a criminal suspect.

There Must Be an Explanation

"As I sat down, they came up and shut the door, and they just went back and just started drilling me with questions like, 'Who are you?'" Fairchild said.

The DNA test results challenged everything she knew about her family. Yes, her boyfriend was the father of the children, and, yes, they were all related, according to the DNA, except for Fairchild. She was told she wasn't the mother.

Fairchild was certain a mistake must have been made, but she recalled a social worker saying to her, "Nope. DNA is 100 percent foolproof and it doesn't lie."

Fairchild was not only denied government assistance for her young children, she was now suspected of possibly acting as a paid surrogate mother and committing welfare fraud. She was in danger of having her kids taken away for good.

Fairchild said before she left, the social worker told her, "You know, we're able to come get your kids at any time."

Fairchild began to panic. She knew they were her kids. So she rushed home to search for photos of her pregnancy and found her children's birth certificates. She told her parents, who couldn't believe the test results.

"I thought she was joking but then she started crying on the phone. I said 'Oh, it's got to be a mistake. I was there when the kids were born. I saw them come out. I held them in my arms, you know,'" said Fairchild's mother, Carol Fairchild.

"I almost went insane inside. I couldn't imagine why if this could happen, my daughter is not a liar," said Fairchild's father, Rod Fairchild.

Fairchild called her obstetrician, Dr. Leonard Dreisbach. He was there for all the births and assured Fairchild he'd vouch for her in court.

"I would have told them that she certainly had these three kids, and that they were hers, and that I don't know what's wrong with the DNA testing, but I know that she had the kids," Dreisbach said.

But none of that seemed to matter, because DNA tests were considered infallible -- the gold standard in court. DNA showed that Fairchild's genetic makeup did not match that of her children.

To eliminate any chance of human error, new DNA tests were ordered from different labs. It was an agonizing wait, but the results were the same: The children weren't hers.

Fairchild knew then that she was close to losing her kids. After three court hearings, she said the judge looked at her and told her to find a lawyer.

Battling in Court

It was another uphill battle in the courtrooms. Most of the attorneys Fairchild turned to were not willing to fight DNA evidence.

Attorney Alan Tindell finally agreed to take the case, but he questioned her extensively about her connection to the children. "These aren't your sister's children? These aren't your brother's children? You didn't abduct these children from anyone?" Tindell said that given how adamant Fairchild's answers were, he decided to believe her.

But Fairchild and her family remained frightened, fearing a knock at the door at any moment. So they made plans to hide the children from authorities.

"Getting that summons in the mail to go to court, that they were trying to take my kids from me, my stomach just went into a big old knot. I just started crying, and I called my family, and I held my kids and was scared," Fairchild said.

"I'd sit and have dinner with my kids and just break out crying. They would just look at me like, 'What's wrong, Mom.' They'd come get me a hug, and I couldn't explain it to them, because I didn't understand," Fairchild explained.

Fairchild was in a tough spot, up against a government that thought she was at the very least a fraud, with foolproof scientific evidence weighing against her.

But then she got a break. Across the country, there was another woman with DNA that didn't match her children's, but in this case, the doctors had cracked the medical mystery.

Another Woman, Same Story

In Boston Karen Keegan had received a chilling phone call from her doctor. It came during a very difficult time in her life, just as she needed a kidney transplant.

Keegan recalled what the doctor said to her: "Mrs. Keegan, we have some unusual news to report to you. We've never had this happen before, but your children don't match your DNA." That revelation came after her family members had had their blood tested for compatibility.

"Any child from a mom and dad should inherit genes from both the mom and the dad. In Keegan's case, it appeared that her two boys hadn't inherited any of her DNA," said Dr. Lynne Uhl, a pathologist and doctor of transfusion medicine at Beth Israel Deaconess Medical Center in Boston. "They weren't hers. So we scratched our heads and said, 'This is really unusual. How can this be?'"

Boston doctors asked Keegan the same type of questions that had been asked of Fairchild in Washington They asked Keegan where her two sons had come from, since their genetic code was not the same as hers.

"They wanted to know the name of the hospital where my children were born. They had some other thoughts, like perhaps this was some kind of in vitro fertilization or even worse, that this woman just might not be completely telling the truth or even be psychologically unbalanced in some way," Keegan said.

Keegan's doctors investigated the case further.

"It was a medical mystery. Certainly there were individuals whom we ran the story by who said, 'There must be a skeleton in the closet," Uhl said.

Doctors took DNA samples from all over Keegan's body. They tested her blood, her hair and swabbed her mouth. Still nothing matched her sons' DNA. But Keegan had another idea.

Keegan told Uhl that she'd had a thyroid nodule removed a while back.

After an extensive search, doctors found a sample of her thyroid tissue saved in a nearby lab in the Boston area. According to Uhl, this piece of tissue was the key to solving the medical mystery.

The DNA that would match her sons' DNA could have been anywhere in Keegan's body. But her thyroid was where she matched her sons' genetic code.

The mystery was solved. In a way, Keegan was her own twin.

"In her blood, she was one person, but in other tissues, she had evidence of being a fusion of two individuals," Uhl said.

It's a rare condition called chimerism, with only 30 documented cases worldwide. In Greek mythology, "chimera" means a monster: part goat, part lion, part snake.

In human biology, a chimera is an organism with at least two genetically distinct types of cells -- or, in other words, someone meant to be a twin. But while in the mother's womb, two fertilized eggs fuse, becoming one fetus that carries two distinct genetic codes -- two separate strands of DNA.

The twin is invisible, but for chimeras the twin lives microscopically inside the body as DNA.

When Uhl told Keegan she was her own twin, Keegan said she was shocked. "You wouldn't imagine that that could even be possible."

Still Fighting to Keep Children

But what did this new discovery mean for Lydia Fairchild, the woman living across the country who'd been fighting to keep her children?

The state was still so suspicious of Fairchild that when she gave birth to another child, a court officer stood in the delivery room to witness an immediate DNA test.

"They took DNA from the baby and myself right then and there, after birth, and it came back that there is no way possible that baby is mine," Fairchild said.

Even though they'd witnessed the birth, officials believed she was acting as a surrogate, possibly bearing a child for money.

Fairchild's attorney was determined to solve the mystery. That's when he came across Keegan's chimera story in the New England Journal of Medicine.

"I asked the judge to postpone the case until these tests could be done," Tindell said.

After the tests were done, there was proof that Fairchild was her own twin as well. The judge finally believed Fairchild was the biological mother of her children and dismissed the case.

"I probably wouldn't have my kids today if they didn't discover her situation. They wouldn't have known to even consider me as a chimera," Fairchild said.

If not for Keegan, Fairchild said she might have lost her family forever.