The Rocket Man hitmaker and Furnish already have a son, Zachary Furnish-John, who was born in December, 2010 from the same surrogate.
The mother, whose identity will never be revealed, is thought to live in California and is believed to have been connected with the pair through the Center for Surrogate Parenting in Encino.
She is set to give birth again next year (13), according to British newspaper The Sun.
A source close to the couple tells the publication, "They are over the moon. Elton and David love this lady like a sister and they feel indebted to her for life. Naturally she has been well rewarded. But her identity will never be revealed."
Sir Elton recently revealed he hopes to have more children and expand his family, insisting he doesn't want his son to grow up as an only child.