“Happy Mothers Day from little W and me! 💜,” the actress, 26, captioned her post, which shows her clutching the baby, fresh-faced and wearing two-toned glasses. She did not provide the baby’s name or sex.
“They’re extremely happy and have been looking forward to starting a family,” the source said of the actress and the New Politics singer, who have been together for three years.
Serratos — who stars as no-nonsense Rosita on AMC’s zombie apocalypse series — won’t be the only mom on set.
Masterson shared her advice for costar Serratos in March, saying parenting is all about “balance.”
“Balance, and having a really good partner who’s there for you and is able to take care of your kid when you’re working,” she previously told PEOPLE.
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The actress added it was tough to return to work after giving birth.
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“It’s super hard being away from your kid, like you don’t quite understand it until you do it and it’s sad and you’re like, ‘Oh man, I’m missing out on so much,’ ” she said. “But I grew up in a household with a super, super hard-working mother and she showed me what hard work gets you. I hope that Marlowe can look at me and admire me for that.”