The Smash star welcomed son Ronan Laine Gallagher on 13 March (17) in Los Angeles.

"We are overjoyed with the new addition to our little family," the actress and singer tells People magazine. "It's incredible how much love you can feel for someone you've just met."

Megan and her husband Brian Gallagher are already parents to two-year-old Viola Philomena.

The 35-year-old announced her second pregnancy in January (17), and opened up about balancing motherhood with her career, revealing she never imagined she would ever become a mum.

"To be honest, separately, neither Brian nor I ever thought we were going to have children," she told Fit Pregnancy and Baby magazine. "I had this idea from childhood that there wasn't room for parenting and my career, and I chose my career. Thankfully, I was totally wrong."

Megan was also surprised about how much she enjoyed being pregnant.

"I didn't have morning sickness either time - knock on wood," she explained. "Before I had (Viola), I thought it was going to be weird to feel a baby moving in my belly. But I love it - even the kicking and punching.

"Near the end of my last pregnancy, my mum said, 'You'll miss her when she's not in your tummy anymore'. She was so right."

Meanwhile, fans expecting to catch a glimpse of baby Ronan on social media will have to wait because Megan prefers to keep her children away from the public eye.

"I don't post many photos of my daughter on social (media)," she added. "I don't judge other people for it, but I feel they're her images to choose what to do with. I'll email pictures to family, which forces me to engage with them instead of hoping they like my post."