McPhee dressed in a black knit sweater and gray yoga pants that accommodated her growing belly. She completed the look with a pair of black sneakers.
Her hubby wore a similarly casual ensemble, opting for jeans and a blue T-shirt.
The pair looked to be aiming for a low profile as they both sported baseball caps and face masks.
The location of the outing was Montecito, California, which happens to be the hometown of good friends Prince Harry and Meghan Markle.
The parents-to-be reportedly grabbed lunch together before heading to a baby store to shop for the little one.
The happy news comes just under a year and a half after the pair's June 2019 wedding in London.
The two first met in 2006 when McPhee was a contestant on the popular singing competition show American Idol and Foster served as her mentor on the series.
Though this is McPhee's first time being a mom, the actress has discussed motherhood in the past, especially regarding her role as a single parent in the former CBS series Scorpion.
In a 2014 interview with Uproxx, McPhee confessed she originally faced serious worries about not being able to accurately portray the deep and complex relationship between parent and child on-screen.
"What my thought was was I don't want people to not feel like this isn't my son... I just had to really focus myself and just say like, 'this is your son' for that," she explained.
"She's never going to give up on him. And so how do you like convey that in such a short amount of time?"
Fortunately, the House Bunny actress confessed she did have some experience to draw on: dealing with her friends' kids.
"I don't really have a hard time kind of taking a motherly role with my own friend's kids and things like that. Sometimes I probably tell them what to do and I probably shouldn't," she joked.
Meanwhile, Foster already has some experience with fatherhood, as he already has three daughters — Sara, Erin, and Jordan — with first wife Rebecca Dyer. He also has two other daughters from previous relationships and used to be stepfather to the Hadid siblings.
McPhee is not the only celebrity to recently announce baby news. As was previously reported by The Inquisitr, singer Meghan Trainor shared she is expecting her first child with husband Daryl Sabara earlier this week.