Unbelievable: She was her own twin. And her twin was the mother of her three children.

In 2002, 26-year-old Lydia Fairchild of Washington state was pregnant with her third child.

She had been in a rocky relationship with the children’s father but, at this particular time, they were not together. Through a series of unfortunate events she had found herself out of work, and unable to support her growing family, so she applied for welfare payments.

The application process was relatively straightforward. It involved means and assets testing, but also a DNA test so that Fairchild could prove that her soon-to-be three kids were hers and, thus, that she was able to claim welfare payments for them. The children’s father also had to participate in the DNA test.

Everything seemed normal, until Fairchild was told that she would be the subject of a possible investigation into welfare fraud.

The DNA tests had come back. And, while the children’s father’s DNA had indicated a parental relationship, hers had not.

She was not their mother.

The news not only greatly confused Fairchild, but also her obstetrician who had watched Fairchild give birth to her two older children and been treating her throughout her third pregnancy.

A re-test was ordered, but the same result came back. She was not her children’s mother. Not even the mother of the child that was in her womb.

What followed was an intense period of questioning, and looming legal action.

Fairchild told ABC News, “I knew that I carried them, and I knew that I delivered them. There was no doubt in my mind.” And yet, a couple of months later – now in a late stage of pregnancy – she found herself in court trying to prove it.

It wasn’t until one of her lawyers stumbled on the case of Karen Keegan that it all started to make sense. Karen Keegan was a 52-year-old woman from Boston, Massachusetts. She required a kidney donor, but DNA tests had revealed that she only shared a genetic link with two of her three sons.

Keegan had a very rare genetic condition known as ‘chimerism’. It can occur when a twin merges with its fellow twin in utero, at a very early stage of pregnancy. It essentially means that a person has two different sets of DNA, the second set normally concentrated in a very small area of the body.

The answer to the Fairchild mystery was evident.

She was her own twin.

Further DNA tests of Fairchild were ordered. Finally, DNA taken from a cervical smear was tested, and matched that of her children. Her twin had lived on only in the cells of her ovaries and, in essence, had gone on to mother her children. The twin’s location in her reproductive area also explained why all three of Fairchild’s children had been born without Fairchild’s DNA. (A rare occurrence for people with chimerism – only one of Keegan’s children was found to have been mothered by her twin.)

The case raises an important point about how we treat DNA testing, especially in forensic situations. As Fairchild’s attorney Alan Tindell pointed out, “People go to death row because of DNA tests, people are released from death row because of DNA test.”

And yet, cases like Fairchild’s show that DNA might not be as accurate as we think.

Scary.