I think this should be an extremely effective ad among people who have, like, a pulse. And maybe emotions. But it's also doing something very interesting, which is targeting evangelical (and other religious) conservatives. Anti-choicers love to talk about children born with disabilities; a child born with a disability is always a "gift" or a "blessing". Obviously they don't actually believe every life is special and sacred, but they like to believe they believe that. So they fetishize children born with disabilities as a way of signaling that they do.
This ad plays directly to that sentiment. The first 20 seconds could easily be taken straight from an anti-abortion ad. It's a slice of conservative-friendly advertising designed to reinforce religious conservatives' uneasiness with a twice-divorced libertine who supports Planned Parenthood. I haven't seen polling on just how vulnerable Trump is among them, but it looks like Mormons at least have their doubts. And Priorities USA is smart to try to capitalize on them.
Religious conservatives aren't going to end up voting for Godless Baby-Killing Hillary (whom the ad doesn't mention). But they might stay home, or vote for Gary Johnson, or write in Zombie Jerry Falwell--anyone but Trump. And losing more than a few of them would be disastrous for him.
Update: Priorities USA has expanded its buy for this ad (scroll way down), which suggests they've seen evidence of its effectiveness.