Katharine McPhee, 37, poses in swimsuit from 40-year-old step-daughter Sara Foster’s collection

She gave birth to her first child five weeks ago.

And Katharine McPhee is already showing off her swimsuit body on her Instagram.

On Thursday, the 37-year-old posed for a mirror selfie wearing a one-piece from her step-daughter Sara Foster’s swimsuit collection. 

‘Best one piece I’ve ever tried,’ Katharine wrote of the high-cut swimsuit.

‘Usually it’s cut too low and the torso isn’t long enough,’ she explained. ‘Straps are adjustable. So comfortable!’  

‘You nail it @sarafoster,’ she added.

Sara is one of five adult children that McPhee’s husband David Foster has from his previous relationships.

The prolific music producer, 71, shares Sara, 40, and her sisters Erin, 38, and Jordan, 34, with his second ex-wife Rebecca Dyer to whom he was married from 1982 to 1986.

His other children are Allison Jones Foster, 50, who was born when he was 20 and placed for adoption only to reconnect thirty years later, and Amy Skylark, 47, from his first marriage to B.J. Cook.

David and singer and actress Katharine married in 2019 and welcomed their son Rennie on February 24.

Earlier this week, Katharine had proudly showcased her post-baby body in a brown one-shoulder bikini and told her social media followers that she ‘loves’ her new curves. 

The photos come after Katharine insisted to People in March that she didn’t feel any pressure to ‘bounce back’ or ‘fit into her jeans’ following her pregnancy. 

‘I’ve just been so grateful and happy that I’ve had a healthy baby and I’ve felt really good, emotionally and physically,’ she told the outlet.

‘I’m so happy that I don’t have this crazy pressure yet to fit into whatever jeans I have in my closet. I don’t even think about them right now!’ 

The new mom added that it’s ‘nice to have a break’ from working out, too. 

She mused: ‘In my everyday life, I would have an event coming up or need to work out and it’s just really nice to have a break and be happy where I am.’