Hilary Duff and Mike Comrie haven't had much alone time since welcoming son Luca Cruz nine months ago.

On Access Hollywood Live Dec. 14, the 25-year-old Lizzie McGuire star talked about how raising a child has decreased her sex drive. "It's definitely different. I'm so exhausted at the end of each day, so I'm like, 'Can this not happen at night when I'm exhausted?'" Duff laughed. "You find different times to do it."

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The couple's son is "into everything," Duff explained. "I'm running around the house just trying to keep up with him. He's so fast . . . He's trying to talk and he's a little aggressive."

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It might be another year or two before Duff and Comrie, 32, decide to expand their family. (The couple tied the knot in August 2010.) "Mike and I are literally so obsessed with Luca. He's perfect! We should just be one and done," the first-time mom giggled. "But I think once your kid starts getting older you miss that baby phase and you go in for round two."

When she isn't looking after her little one, Duff is working on getting her post-baby back in shape -- and it's paying off! Though she recently slipped on a pair of size 26 jeans, the singer said she's "trying not to obsess" about exercise.

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"I am worried because everyone wants their body back. It's all in time, you know? It took nine months to get there, so hopefully it won't take that long to get it off," Duff said on Access Hollywood Live. "But I'm doing it slowly."

This article originally appeared on Usmagazine.com: Hilary Duff Talks Sex After Baby: "It's Definitely Different"