Check your local parks and recreation facilities. DC used to have some good ones, although they're all during the workday now. I've seen them at the YMCA as well.

Not specifically prenatal but there are water aerobics classes at the Washington sports club in Columbia heights, DC

I am looking for prenatal water fitness classes near DC. I am willing to drive to nearby areas but I can't seem to find a pool or gym that offers them. Any thoughts? Thanks!