Verty touchy/sensitive issue. Logically, there are a LOT of reasons as to why certain persons should NOT be having babies, and I'll go as far as having babies is not, in my view, a "right" (in the sense that some think that EVERY SINGLE PERSON should have babies), because once one has a baby, one has "duties" towrds that baby AND society, and some people just can't cope with those.
BUT, that's all an "intellectual" reasonning, based on economics too. But human rights say the exact opposite, that everyone should be allowed to have kids and that society should provide for ALL its children. Even drawing lines such as limiting welfare to people with a certain amount of children is discriminatiry, because what you're saying is that poor people should not be having kids: are you implying that because one is poor, one is "less" of a good parent? that's extremely insensitive, and false too. So it's a very hard line to draw.