05/02/2011 at 01:00 AM EDT
Sure, Mariska Hargitay and her husband, Peter Hermann, had been discussing adoption for years. But when it came time to decide on expanding their family, there was one driving force behind the decision: their 4-year-old son August. "He would come home from school and go, 'I want a baby sister! I want a baby sister!'" says the Law & Order: Special Victims Unit actress, 47. "We knew we were eventually going to adopt, but he takes full credit for it!"
Of course, baby Amaya Josephine, adopted in March from within the United States, was a dream come true for Hargitay as well. "I have a hope chest, and I'm not kidding you-for the last 15 years, every time I see something special, I'm like, this is for my baby girl!" says Hargitay with a laugh. "Let me just say, this kid is hooked up!"
Mom's admittedly got it pretty good herself. "There's this feeling of surprising calm, like this was meant to be all along," says Hargitay, adding that she and Hermann, 43, plan to try for more children. "I just feel like she's home."