Like I said, I'm not sure if births respect the pop cap. Like, you might keep the baby/child setting to the default meaningless 100:1000 but with a cap of 0 dwarves may simply not breed anyway. (You can still potentially get children as migrants; I don't think I've ever seen a baby arrive with its mother on a migrant wave.)
Also, the pop cap setting is somewhat misunderstood/buggy. AFAIK the only way to absolutely-kind-of ensure that you don't get extra migrant waves beyond the first two is to set it to 0. In my reading about it, players seem to misunderstand how it works; they treat it as purely a performance feature independent of any gameplay mechanics, when what appears to be the case is that it still has to go through the "communication" simulated between your fort and the mountainhome by the liaison. So you get a wave in summer, fall, and possibly the next spring depending on how long the liaison took to finish his meeting.
Also remember that while the first two waves are guaranteed, the size of them varies and could still result in a total of 40 dwarves or more.