What I love most about fatherhood," British pop star Seal says about caring for fiancée Heidi Klum's 10-month-old daughter Leni, is "the opportunity to be a part of the development process of a new life." Looks like that opportunity will come knocking again soon. Sources confirm that the German supermodel is expecting a baby by the fall. "They are thrilled," says a pal. "They have created a non-traditional family unit."

By all accounts, that unit has been inseparable since the couple got together in February 2004, shortly after Klum split from Leni's father, Italian businessman Flavio Briatore, 54. From the beginning, Seal, 42, has prided himself on playing the proud papa—from celebrating with champagne at the New York City hospital where Klum, 31, gave birth last May to dedicating his latest album, Best: 1991-2004, first to "my daughter Leni," then to Klum and her parents. Says the pal: "He is crazy about Leni, he is always carrying her around. He is the first one to change a diaper." The never married singer—who has no other children—is obviously committed: The day before Christmas Eve he proposed to Klum atop a Canadian mountain, and the couple have purchased a $4.3 million Bel-Air estate. And when will they finally seal the deal? "I would say they'll be married within a couple of months," says a source. "He takes it very seriously. Heidi and the baby come first." Make that the babies.