Prenatal Aerobics & Fitness: Staying Active During Pregnancy

Question
I am 28 years old and have never been very active.  Recently I have been sporatically walking and doing some low impact aerobics.  Suprisingly, that has allowed me to lose 30 pounds.  I just found out that I am 5 weeks pregnant.  I want to continue to excercise because if I stop now, I know it will take me a while to start up again if at all. I have looked around for prenatal classes and all that is being offered in my area is yoga.  I rather have an aerobics class.  Is is safe to take a regular aerobics course? Do you know of any programs, gyms, clinics, etc. that provide prenatal classes in the DC/MD/VA area?

Answer
The good news is that since you are already acclimated to aerobic exercise, you can continue your regular workout until about 20 weeks or so. Studies show that women who participate in regular aerobic activities in pregnancy grow larger placentas and have strong immune systems, healthier babies and few complications than more sedentary women. So a major high-five to you for establishing a fitness routine before conception. This is ideal.
At 20 weeks the placenta stops growing, so at this point you should start to slowly taper off the intensity of your aerobic exercise. Stick with low/non impact. Stay in a range that you perceive as "somewhat difficult" and where you can speak normally, no panting.
While I don't know of any specific programs in your area, many local YMCA's and larger health clubs have special programming for pregnant women. In the last trimester, many women enjoy swimming, which takes the pressure off the lower back.