There's a thread on the front page right now from the "two x chromosomes" sub. In it, the original poster says as follows:

My ex-husband was never very nice to me while we were married and certainly not after we got divorced last year. Part of the reason we divorced was because I wasn’t ready to have children and he was. Eventually, he started drinking more, demanding sex, and I had enough, opting to leave him before it got any worse. I met a new man soon after and I’m happy to say that after a year of dating, we are engaged! Last month, while planning for the wedding, I discovered that I am pregnant. It was shocking but we are rolling with it, the baby is due a few months before the wedding so we aren’t changing our timeline.

Looks to me like what happened is that this woman started an adulterous affair while still married; denied her husband sex; and ultimately dumped him for a new man.

But in her mind, the previous husband is the bad guy because he "was never very nice to me"

Truth be told, the first husband dodged a major bullet here since his wife dumped him rather than engaging in paternity fraud.

The post goes on to talk about how the first husband opened a credit card in the woman's name. Which is admittedly a lousy thing to do but not nearly as bad as what the wife quite likely did.