As I said on the other thread I know someone who fostered her nephews when she was 19, under this she will not be able to have her own child because she has not much chance of an education or a good job with 3 under 8s.
The rape exception is not workable for 100s or reasons. Taken abroad in a forced marriage and be allowed 'back' when you are expecting number 3, all a product of rape, but possibly not in the country where you were married.
Other situations, married couple have a child, then husband has affair and gets mistress pregnant, so can wife have another child?
Or adopting a child, but the child is part of a 'sibset'?
Situations where a child has a baby, say a 15 year old has a baby then in a benefit claim the 15 year old is treated as a child of the family, and their baby is too. So if the 15 year old has a sibling then there will be no benefit for the baby.