Wow, I've never thought about this situation before. While I don't know the rules of every sorority, from what I understand, it is acceptable for a woman to be married and in a sorority, and I would assume that if no rules are made against this, then the organization must understand that it is possible that a married woman might get pregnant during their time in college. So I suppose if married women are allowed, they wouldn't make a rule against something that...well, often happens to married women?
In terms of the chapter's decision, I don't think that it should be a problem. If the girl is 100% sure that she is not going to either drop out of college at some point to take care of the child herself, or transfer to be closer to the child or the child's father, etc. then I think the fact that she is a mother shouldn't reflect badly against her. It could be a positive in that it would bring some diversity to the chapter (not saying the chapter is lacking; I have no idea. Just saying that it's a different viewpoint on things).