Hilary Duff is experiencing "wonderful bonding time" with her newly expanded brood.

The actress enjoys doting on her four-week-old son Luca with her loving husband Mike Comrie.

Despite the inherent challenges of caring for a newborn, first-time mother Hilary relishes every second she spends with her baby boy.

"It's such a wonderful bonding time," Hilary told People magazine.

"Just to have your baby thrive and knowing that you're doing that and you're responsible it's such a rewarding feeling."

Hilary is enlivened by every aspect of being a mum.

"The day is exciting if he makes a new face or he gets to take a real bath instead of a sponge bath," she gushed.

Each benchmark Hilary encounters as she dotes on Luca blows her mind.

"You know what was a big milestone? The belly button falling off," she said.

"That was a huge one. And then I wanted to keep it but my husband was like, No, that's gross, you can't keep it!' I'm like, I'm totally keeping it,' and I secretly kept it. But now it's not a secret anymore. It's in a Ziploc bag in the back of my makeup drawer. Luca's going to think I'm a freak!"