A surrogate mother from California, who gave birth to two babies, found out one of the children was biologically hers.
Jessica Allen became pregnant with her own biological child while she was carrying another baby as a surrogate for a Chinese couple.
Mrs Allen said doctors first thought the second was an identical twin.
As a result, both babies were given to the other woman after delivery and the Allens had to fight to get back their biological son.
Mrs Allen had already given birth to two healthy children, but had decided to become a surrogate mother.
She told Newsday on the BBC World Service she had ovulated despite being pregnant.
"I was pregnant through IVF already and at about six, seven weeks they found another embryo and they labelled it that the one embryo that they transferred split and turned into twins.
"My body naturally still ovulated while I was already pregnant - that's very rare - no one really knows about it but obviously, with my situation, more people are learning about it.
"It's called superfetation - it has a medical term to it, so why this process is not explained to surrogates, I don't know. They don't give this as a possibility."
'We definitely want our son'
Mrs Allen said she did not see the babies when they were born last December, but the other mother then texted her pictures of the babies when they were a few weeks old.
The intended mother also said she was waiting for Mrs Allen to feel well before revealing that she had doubts about the origin of one of the babies.
"I immediately freaked out and I asked my case worker, 'what's going on, how can this be, what's happened, how did this happen?' and she didn't have any answers for me," Mrs Allen told Newsday.
"So she [the case worker] said the next thing we need to do is to get me to have a DNA test."
The intended mother also took the baby for a DNA test and within a fortnight asked Mrs Allen for a Skype call.
"That's when she announced that the results indicated that I was his genetic mother," Mrs Allen said.
"During the Skype call she did suggest that we could give him up for adoption and they already had a couple thinking about adopting him.
"At this point we had no idea what to do, so by the end of the call we told her we were going to talk about it, because we needed to figure out how we're going to get ready for a baby literally overnight."
However, the next day Mrs Allen told the woman: "We definitely want our son."
Not on the birth certificate
But Mrs Allen and her husband ran into difficulties because - legally - they were not the parents.
They were told that because the intended mother signed the baby's birth certificate, she was his legal mother and then if she wanted to give him up for adoption, she could
"I'm not on his birth certificate but I am his genetic mother, my husband is his genetic father and we have that proof through DNA," said Mrs Allen, who added she and her husband were also asked to pay back part of the fee to the couple because they had paid her to carry more than one child.
They had also incurred fees for a lawyer they had hired to help them get custody of their son.
The Allens were finally reunited with their baby boy, who is now 10 months old.
When she met her son, Mrs Allen said she was a nervous wreck.
"When she [the case worker] pulled him out of her car and walked towards me, I just snatched him from her and said 'give me my baby' and I was just kissing him and trying to look at his face for the first time.
"I sat in the back seat with him so I could see him and talk to him and stuff."