I don't have any info about maternity stores in the Chicago area, but I just wanted to say Chicago's a wonderful place and I hope you get to see some of the sites while you're attending the conference. Hopefully Sandi will have some ideas for you (she lives quite near and used to work there ~ she's also a wonderful tour guide; we had the greatest time on our Chicago trip!)
Thanks for letting us know about your absence in advance so we don't get to fretting.

Have a great time and good luck with the maternity shopping.