You can choose to have a baby or not, and I don't see why you should get special treatment at work because of that decision.
I understand meritocracy is an illusion at best, but it's an important illusion to maintain. If someone is pulling less weight, they should receive less compensation. They certainly shouldn't be rewarded for taking weeks or months off to work on on side projects, which is essentially what a baby is.
But maybe I'm just being harsh by thinking of children as side projects, and I should be more sensitive. Fine, let's say that having a baby is a medical condition. For any other medical condition, your options are burn all your PTO or go on short-term disability. Why should having a baby be any different?