When Mary and partner Melanie Rickey wanted to conceive through IVF, they had to choose a sperm donor.
But rather than choose an anonymous donor, which they previously claimed they did, the red-headed star has now admitted it was her brother, Lawrence Newton, who helped conceive their child.
Speaking to Times magazine, Mary told them he was honoured at being asked to donate sperm for the IVF treatment, and even attended the birth of the child two years ago.
"Lawrence was the first one there (with us) and he picked him up and it was just very emotional.
"I said to him ‘thank you’ and he said ‘it’s my pleasure’ and we just held each other."
She added: "And now when I look at Horation, he is a complete mix of (Ms Rickey) and me."
Mary and Melanie were one of the first couples to have their civil partnership converted into a marriage when the law came into effect in December, doing so just one minute after it was legalised.