The Fayetteville and Hope Mills YMCA's have Yoga too. But no pre-natal that I could find.
Westover Recreation Center has Yoga classes
Lake Rim Recreation Center has "Fit to Deliver" pre-natal low impact exercise classes
Here is a list of classes the Fort Bragg MWR has
http://www.fortbraggmwr.com/sportsrec/aerobicsclass.pdf (broken link)
and the Pope AFB MWRs class schedule
http://www.43fss.com/frontimages/Fit...r_2009_WEB.jpg
Lots of yoga but nothing specifically "prenatal" that I could find.