An American woman has astonished medical experts by giving birth to twin boys - with different fathers.

In a case known as heteropaternal superfecundation, 11-month old Dallas-born twins Justin and Jordan have different dads, a phenomenon so rare there are only a handful of documented cases in the world.

Mia Washington, who has admitted having an affair with another man at the time the twins were conceived, first decided on a paternity test after noticing the boys had different facial features.

Her fiancee James Harrison is the biological father of Jordan, but Justin's biological father is the man Washington had an affair with.

Harrison said he will bring up the two boys as his own; he said he has forgiven his fiancee and promised to stay with her, according to online reports.