AITA for deciding my ex didn’t need to know I’m pregnant?

I (28F) was married to my ex-husband for six years. During that time, we struggled to conceive. After about four years, things took a dark turn—my ex was under pressure at work and, instead of talking to me, he cheated with a coworker. A week later, they both sat me down to announce she was pregnant.

Like a fool, I stayed. I believed in him, in love, and honestly, I felt like I had no one else. His family treated me like one of their own, but once the baby arrived, everything changed. She became the center of their universe, and naturally, the mother—his mistress—was suddenly a permanent part of our lives.

She was everywhere: every family photo, every event, every holiday. I started to feel invisible. Worse still, I always suspected she had feelings for him and wasn’t shy about flirting. People assumed she was his wife. The final straw came at the baby’s first birthday party—his family handed me the phone and asked me to take a picture… of him, the mistress, their daughter, and his parents. I wasn’t even asked to be in the photo. That’s when it hit me: I had become the outsider in my own marriage.

I filed for divorce shortly after. It was brutal, but I picked myself up, focused on my career, got promoted, and eventually met a kind, loving man. Fast forward to today: I’m in a relationship with someone who truly values me—and we have a beautiful baby girl together.

Here’s the kicker: I got pregnant only three months into our relationship. After years of failed attempts with my ex, I thought I was the problem. But clearly, I wasn’t. It made me wonder if he was the one who couldn’t have kids, and maybe his mistress had lied about the baby being his because she didn’t know the real father.

I never brought it up—it wasn’t my place anymore.

That changed when my current boyfriend posted a picture of our baby on Facebook and tagged me. My ex’s sister, who I was still friends with on there, saw the post and called me. She was shocked and said she wished I had told them I could actually conceive—that maybe it wasn’t me after all. Long story short: they pressured my ex into taking a paternity test. The child? Not his.

Turns out the mistress was seeing another man at the time of the affair and didn’t know who the real father was. She chose to let my ex believe the child was his because she had feelings for him and thought he’d be the better dad.

Now, my ex is furious with me. He says I should have told him when I got pregnant, that if I had spoken up, he wouldn’t have spent years raising a child that wasn’t his. He’s a mess, and honestly, I do feel guilty. I didn’t expect it to blow up like this.

UPDATE: I’ve seen comments saying this sounds familiar or like a repost. This is my first time posting on Reddit. I didn’t come here to paint my ex as some villain—I came because I spent years with him and his family. They were my family too. We promised to stay in touch after the divorce, but after everything, I just couldn’t.

They’re now furious with me—not just for leaving, but for being “complicit” in letting my ex bond with a child who wasn’t his. They feel betrayed.

Another Update: I’ve avoided most of their calls, but recently my ex texted me apologizing for overreacting. He admitted he feels guilty for prioritizing the wrong people and neglecting me when I was actually family. I replied only to ask how the little girl was doing. Even if she’s not his biologically, she loves him deeply, and that kind of bond doesn’t disappear overnight.

He told me he’s no longer financially responsible but still plans to see her monthly—he feels morally obligated. Honestly, my heart breaks for her. Through no fault of her own, she’s lost her sense of family—her dad, her grandparents, her cousins. She’s the true innocent in all of this.

Here’s the input from the Reddit crowd: