I just read that yesterday. If you're pregnant by your ex, wait until after you've aborted to tell them if you absolutely can't keep it to yourself because they can't do a fucking thing about it after the fact. If you tell them beforehand, there will be guilt tripping, stalking, verbal and/or physical abuse, threats of lawsuits and so on. Don't put yourself through more drama, getting an abortion is hassle enough.
If need be, keep the knowledge in your arsenal to use for taking cheap shots at them later. Nothing pisses off an abusive ex-boyfriend like finding out his woman "killed" his "child." Not that he wanted to be a parent or raise a kid, but because a kid is a good way to keep a woman under their thumb indefinitely and he can use the kid to fuck with her by demanding visitation and all that. My ex did shit like that, he was in a relationship with a manipulative woman who told him she aborted after she left him (I don't think it was true because she waffled between "you're gonna be a dad" and "I aborted your son" for like six months) and he went on social media to tell everyone what an evil bitch she was for "murdering" his son. I don't think he knew the gender, he just really really wanted a boy and assumed that's what the imaginary fetus was. He was desperate for a kid, but didn't seem to care that the possible mother was a lunatic. Yeah, quality control was never his strong suit.