Media playback is unsupported on your device Media caption Irina and Anya say they are now the best of friends

Two Russian families are united by a terrible event more than a decade ago. Their newborn daughters were accidentally mixed up in the maternity hospital and grew up with the "wrong" parents.

In a tiny flat in the Ural Mountains, Yulia Belyaeva and her 12-year-old daughter Irina are looking through family photos.

One of the pictures shows Irina as a newborn baby swaddled in a blanket. It was taken the day mother and daughter left hospital. But 12 years on, Yulia Belyaeva has discovered that the baby she'd taken home - the daughter she'd thought she'd given birth to - is not her child.

"I found this out when my ex-husband refused to pay maintenance," says Yulia. "I took him to court to prove that he was Irina's father. We did all the DNA tests. But the results were a total surprise. Not only does my ex-husband have no biological link to Irina - neither do I."

Police believe that on 17 December 1998, there had been a terrible mix-up at the local maternity hospital. Two babies had been given the wrong name tags - and the wrong parents.

"At first I thought it was a joke," recalls Yulia. "Then I couldn't stop crying. My whole world had turned upside down. I kept worrying what Irina would say. And I kept thinking about my real daughter. Maybe she'd been abandoned. Put in an orphanage. Or perhaps she was begging on the streets."

How damaging is it to a baby? Babies are born quite helpless and need to identify who the person is to feed and protect them. They recognise their mother's voice from what they have heard in the womb. They also know her smells. Another study found they recognise the mother's face after a few hours. If the baby is given to another mother soon after birth, that survival instinct is then transferred to whoever is the new care-giver. It would not necessarily be traumatic. There would be a much more severe impact after a year, when they would have formed a much deeper attachment and the disruption of that bond can have life-long consequences. Discovering that mistake aged 12, when you are just developing a sense of identity, would be a big challenge.

Desperate to find her, Yulia went to the police and they launched a search for her biological daughter. Within weeks they had found her.

In a village half an hour's drive from Yulia Belyaeva's flat, lives 12-year-old Anya Iskanderova. In a meadow opposite her house, she shows me her favourite cow April. Anya is the girl Yulia had given birth to. She is the spitting image of her biological mother.

In the house is Naimat Iskanderov - the man Anya thought was her father. Naimat is from Tajikistan. He had married a Russian woman, but they had divorced. It was Naimat who brought up Anya and his other children as devout Muslims. When police told him about the mistake at the maternity hospital and that Anya was not his daughter, to begin with he refused to believe it.

"Then the detective showed me a photo of the other girl, Irina, the one they said was my real daughter," Naimat tells me. "When I saw her face, it was like seeing myself. My arms and legs began shaking. It was awful to think that my child had grown up with another family. And that I had brought up someone else's daughter."

The two families meet regularly now. But the parents admit there is tension between them.

Image caption Yulia and Naimat have raised each other's biological daughters

"It is difficult," concedes Naimat. "One family is Christian, the other is Muslim. We have different traditions. What I fear most is that the daughter I've raised will start going drinking in bars, that she will stop praying and working. I'm worried she will lose her religion."

"There is tension between the adults," says Yulia, "Naimat doesn't like some things that go on in our family, I don't like some things in their home. Both of us are used to life as it has been. Not as it is now. Now it is a nightmare."

More than anything Yulia fears that both children will desert her. She can see that the daughter she brought up is keen to spend time now with her biological father. And the child she actually gave birth to is like a stranger.

'I try to show Anya motherly love," Yulia says, "But she doesn't accept it. She's been brought up differently. She's not used to tenderness. We don't really understand each other. When your own daughter looks at you like a stranger, that's so painful."

Other baby-swap cases 1973: Spanish twins were separated in a Gran Canaria hospital and sued 35 years later

Spanish twins were separated in a Gran Canaria hospital and sued 35 years later 1978: Kimberly Mays Weeks is now a mother herself after being swapped with Arlena Twiggs in Florida, US

Kimberly Mays Weeks is now a mother herself after being swapped with Arlena Twiggs in Florida, US 1989: Robyn Dawkins and Gavin Clinton-Parker, near Johannesburg, South Africa

Robyn Dawkins and Gavin Clinton-Parker, near Johannesburg, South Africa 1995: Callie Conley and Rebecca Chittum in Virginia, US

Callie Conley and Rebecca Chittum in Virginia, US 2006: Two Czech babies were mixed up and returned a year later

Both families are suing the Kopeysk Maternity Hospital for more than $300,000 in damages. Its chief doctor went on Russian TV to apologise for the mistake, but argued the hospital could not afford to settle such a claim. Prosecutors are considering bringing criminal charges against the hospital staff responsible for the mix-up. Although that seems unlikely, considering so many years have passed.

For now the two girls say they do not want to swap parents. They are just glad to have found each other.

"To begin with we were a bit shy," Irina tells me, "but now we've become the best of friends."

"What I'd like," says Anya, "is for all of us to live in one big house."

Irina and Anya were born 15 minutes apart. Now the truth about what happened in hospital has brought them together.