Ditto my collar-popping homegirl.
If you have a chapter at a commuter school with a student body that is not the normal 18-22 bunch, you're going to have all kinds of things that don't go on at "traditional" schools. This is one of them. If you are at a very traditional school where the tongues wag if you pledge a junior, I don't think the mom-to-be is going to be a sister there.
However, I don't think it's up to any of us to judge and say "a pregnant/married/mother should not do this or that." Everyone conducts their life differently. Some single traditional students can't handle being a sorority member and nothing else - some women can juggle 2 kids, a house, a hubby, school and extracurriculars with no trouble.
May I also add this is why we have a pledging period. If ANY woman finds she's bitten off more than she can chew, she can drop out of pledging.