I think that Brooks is deliberate in not rhyming all of the lines in the poem. Consider the first line where we see a breaking of the rhyme scheme:
I have heard in the voices of the wind the voices of my dim killed children.
This line breaks the rhyme scheme, making it of particular note. The language in it is worthy of such a distinction, as it is one of the first moments where there is a profound haunting at what has been done. The "voices" of these children speak, and in describing their speaking, the rhyme scheme is broken. Perhaps, the implications here are that the voices of the children that the mother must reconcile are dead are ones that lie outside what others will ordain as part of the construction of reality. The voices of these children lie beyond such notion, such reality, such a rhyme scheme. This might be where we see the other two lines go outside the rhyme scheme. In constructing reality, the pain and incalculable suffering of the families where abortion is something where standard rules do not apply, and some level of breaking this order has to be taken as reality. In this, Brooks' discussion is one that represents the undeniable and yet unquantifiable pain of suffering that exists between the mother and her own actions of the past.