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The Smash actress, 37, who married the musician and producer, 72, in 2019, gave birth to son Rennie David earlier this year.
Foster drew criticism after he shared a photo of McPhee in a black bikini captioned “What baby!”
McPhee, who has previously spoken about her experience with eating disorders, responded to the “haters” who took offence on Thursday, telling them to “get a life”.
Sharing a photo of herself in a red swimsuit on Instagram, she said the response is “so dumb”, adding: “I’m sorry but we are not sorry. And for all of you who can’t deal with it maybe this helps.
“I’ve struggled with weight my whole 20s and 30. I've gone up and down 10 times over. Does that make you feel better? There is nothing wrong with that - most people do.
“I lost this baby weight without dieting. Zero. Diets are the worst and I've had zero pressure from anyone.
“I've let me body do its thing and found a great workout. That’s it. And guess what? I'll probably gain weight again at some point too. Who cares?
“BUT people freaking out about what my husband captioned seriously get a life ... Stop being so offended by what people post who have zero impact on your life and move on.
“Maybe you should have more of an attitude of like, let’s say ... ‘oh that's nice he thinks his wife is hot’.
“I cannot with this overly sensitive society right now. But as Taylor Swift said haters gonna hate. Byyeeee.”
McPhee and Foster married in London after McPhee finished a run in the West End in the lead role in the musical Waitress.
Responding to McPhee’s Instagram post, her step-daughter Erin Foster wrote: “If I can accept my step mom looking like this, you certainly should be able to. The photo wasn’t altered or filtered and she’s had no surgery.
“You can’t shame someone for feeling cute after a baby whether she’s sharing stretch marks or a six pack. Let her live.”