Liu says he started to be suspicious when the smaller baby turned out to have a different blood type, along with the fact that it doesn't look like him. Liu had his own and the two babies' DNA tested - and discovered that he was the biological father of only the bigger twin.
"My wife admitted that she'd had sex with her former boyfriend after we got married," he said.
Doctors explain that it's quite rare, but possible, since a man's sperm can survive for 72 hours in a woman's body. Link