Fox, 29, has yet to confirm who the father is, however, sources tell PEOPLE that it is ex Brian Austin Green and the pair are now working on their marriage.
The actress is already mom to sons Noah Shannon, 3½, and Bodhi Ransom, 2, with Austin Green. And although Fox filed for divorce in August after five years of marriage and 11 years together, the two have remained friendly.
And while Fox's career is busier than ever, with her promo tour for her upcoming film Teenage Mutant Ninja Turtles: Out of the Shadows and a recent guest stint on the hit sitcom New Girl, the actress' main priority has always been motherhood.
Musings on MotherhoodAfter welcoming her first child, Noah Shannon, in September 2012, she said she was blown away by the connection she felt to him.
"It's exhausting, but it's amazing and you can't – until you have kids – imagine how much you could possibly love a human being," she said. "It's really intense and really overwhelming.
Shortly after giving birth, Fox made the decision to put her career on hold to focus on being a mom.
"It's very hard for me to do this stuff because I feel like this isn't my job anymore," she told Marie Claire. "My job is to be with him. All I wanted to do my whole, whole like was have a baby and, now, I've finally done it. I just want to give Noah as much of myself as I can."
But when she was ready to return to the big screen, the sultry star said she would keep her son in mind when choosing roles that required her to show some skin.
"I'm going to be more cautious about choosing films because I'm already thinking about when he's in school and his friends are going to be showing him my photo shoots with me in a bikini and he's going to be horrified," she explained. "So that will deter me from making some of the choices I made before."
Less than a year after giving birth to Noah, the Transformers star revealed that she was pregnant with her second child. Fox welcomed her second son with Green, Bohdi Ransom, in February 2014.
And while having more kids was already a dream of Fox's, she was the first to admit that having two kids under two was a bit of a challenge.
"As a mom it's hard because I don't feel like I'm every giving either of them 100 perfect of my attention of 100 perfect of myself, so I carry a lot of guilt," she said. "Do they understand how special they are and how much I love them? And are they understanding that they're unique? It's hard to make each one feel like an individual when you have to raise them together and manage them together all the time."
Still, Fox said she hoped having two kids – and a third on the way – so close in age would allow them to share a strong bond.
"I'm excited for the future, to see them be brothers and be best friends and I know that there's gonna be lots of fighting, but there's also gonna be lots of hugs and kisses."
Despite separating in June, Fox and Green, 42, have continued to spend time together. While they've reunited to be with their two children, they've also been seen running errands together without the kids.
In November, the two were spotted heading into a Hollywood café, but left before grabbing a bite. Just months later in March, they were spotted in Los Angeles together on two separate occasions. During the outings, the former couple dressed casually and seemed relaxed around one another.
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In August, the two confirmed their split, with Fox filing for divorce just days later. The actress cited irreconcilable differences, with the date of separation listed as June 15.
"Anything could happen in the future, but for now they've decided it's best to take some time apart," a source told PEOPLE at the time.