I think the reason a lot of chapters would prefer an active to go alum once she becomes pregnant is b/c no matter what the circumstance, it looks "bad/funny/weird" to have an unmarried active sister 8 months pregnant standing out side the university center sporting her letters. Recruitment is 365 days a year. And people talk.
Although I think this is a stupid reason, I think it does have some to it (at least it does on our campus).
edited to change 375 to 365... Hey! It was late and I was tired!
