The 44-year-old actor said on Wednesday's episode of his "...with Brian Austin Green" podcast that his relationship with Fox takes hard work.
"Marriage is hard," Green said. "It's work, I think for anyone. I think when you get to the point like we have, where you have kids and you've been married for a while and we've been together for a long time -- you just take it day by day."
"You're in it and then if you're not, then you look back and you go, 'That was a great 13 years,'" he added.
Green and Fox announced their intent to divorce in August 2015, but confirmed in April 2016 that they were expecting their third child.
The couple subsequently reconciled, and welcomed son Journey in August 2016.
"I try and live day by day and just take it for what it is," the actor said.
"Some people look at divorce or things not working as, like, a disappointment and it's not. The fact that it worked at all is positive."
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"We have three amazing kids," he added. "We've had and have a great relationship and you know, we're just taking it day by day ... To be in a relationship takes work, and it's not easy."
Green and Fox also share 4-year-old son Noah and 3-year-old son Bodhi.
The actor, who is also parent to son Kassius with ex-girlfriend Vanessa Marcil, told Hollywood Pipeline in August that he's open to having a fourth child.
"I want a girl. I really want a girl," he qualified.
"I'm worried that if I try I'll just get another boy. Not just, because I love my boys, but I'll have another boy and I don't know if I'm ready for five."
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