Twin boys with different skin color have been born to a mixed-race German couple, a once-in-a-million occurence according to doctors.

The twins were born by Caesarean section on Friday, July 11, but they were kept from public until Wednesday, German television reported.

"Yes, I did not believe it, almost until now," their mother Florence Gerth said through translation. "They are five days old now and I'm still looking at them and asking myself, 'Are they my children?' It is unbelievable, it is unbelievable. And I thank God, that it's me, because I've heard that the odds are one in one million," she added .

Stephan Gerth, the twins' father said the only thing that matters is "they are healthy and lively".

"This is the most important thing. If white or black, this is of second-rank in this case," he said.

Ryan has lighter skin than his twin Leo. Their mother, Florence originally comes from Ghana and their father, Stephan is from Potsdam near Berlin. They have been married for eight years.

Mixed race women and men have genetic codes for both light and dark skin, according to experts. Doctors believe the Gerth twins inherited entirely different genetic codes from their parents.

The occurrence is not unprecedented. In 2006, British twins Layton and Kaydon Richardson were born to a mother of English and Nigerian heritage.

"As they were gradually getting older, Kaydon started to change color, got darker and Layton, his hair started to go fairer," said Kelly Richardson, the twins' mother. "And then it was like when we went out in public people really started pointing it out."

Mixed race women and men have genetic codes for both light and dark skin, according to experts. Doctors believe the Gerth twins inherited entirely different genetic codes from their parents.