Foster was also among the many famous figures who excitedly commented on the pic, writing, "My most AMAZING and BEAUTIFUL GLOWING wife!"
A source confirmed the news of McPhee's pregnancy to ET on Oct. 8, sharing, "Katharine and David are expecting their first child together and the couple is very excited. [They] discussed having kids early on and were on the same page."
This is 36-year-old McPhee's first child, while 70-year-old Foster has five daughters -- 34-year-old Jordan, 38-year-old Erin, 39-year-old Sara, 46-year-old Amy and 50-year-old Allison -- from previous relationships.
The former Smash star and the legendary record producer got married in June 2019. It's the second marriage for McPhee and the fifth for Foster. McPhee was previously married to Nick Cokas from 2008 to 2016, while Foster was previously married to B.J. Cook, Rebecca Dyer, Linda Thompson and Yolanda Hadid.
For more on Foster and McPhee's romance, see the video below.