In this poem, the speaker is a woman who has had multiple abortions in her life. She says that one does not forget "the children you got that you did not get," the children one conceived but did not carry to full term. She seems to recall in some detail what the "damp small pulps" looked like when they were aborted, whether or not they had little or no hair, and so forth. She has "heard in the voices of the wind the voices of [her] dim killed children." She has thought about the jobs that they will never do, the ghosts that they will never fear, the thumbs that they will never suck. She says that "If [she] stole" their births and tears and loves and deaths, "even in [her] deliberateness [she] was not deliberate."
In other words, it seems that she did not have an abortion in order to take things away from them; this part of the process was not deliberate (though some other aspect of it must have been). She struggles with how to think of them: they are not "dead"—in a way, they were just "never made"—but in another way they were; each was "born, [...] had body, [and] died." They just didn't really do anything else that one does in a life. In the end, she says that she "loved [them] all."
The second part of your question can really only be answered with a personal opinion. This woman feels that she "loved" all of "the children" she aborted, that she "knew [them], though faintly." If a mother has a child and that child dies, we would still consider her a mother. I suppose, then, that it is possible to consider a woman who has only conceived a child to be a mother. On the other hand, perhaps it depends on whether or not you feel that an aborted fetus is a child. Perhaps that question depends on how old the fetus is; perhaps not. There is lots to consider in regard to motherhood.