There's one right in Catonsville
and the Yoga Center of Columbia has classes, too.
I know Ojas wellness/yoga center has prenatal yoga
Also any gym with a pool has "classes" that any pregnant woman can join... As a former swim instructor/pool manager/water aerobics instructor...it is safe to do lap swim programs or any water aerobics program when pregnant- Plus you feel weightless in the water which cuts down on injuries!
I'm starting one in a couple weeks at Baltimore Washington Medical Center (old North Arundel). They offer a prenatal yoga class thats $40 for 8 weeks.