I'll be 18 weeks along tomorrow and I had asked my doctor about exercise at my last visit. She said that aerobic exercise is good as long as it's not too strenuous. She said to make sure that I listen to my body and only do what I feel comfortable with. With that in mind, I have been doing (until I get up the nerve to do more) fast paced walking for about 30 mins. But I plan on starting working out on a stationary bike and adding some light weights. I don't feel comfortable doing any sort of abdomenal exercises, so I avoid them.
I had lost 20 lbs before I found out I was pregnant. I remember working out pretty hard the day before I found out. I was hoping that I hadn't hurt the baby. But the baby and I are just fine.
Exercise is supposed to be very good for you and the baby! It also helps shorten the length of labor....And i'm all up for that!!!
Good luck!

I have been pregnant four times (14 weeks now) and have only excercised religiously with my second baby. Not doing to well now, kinda lazy, but anyway, with my second my pushin was non-existent and I had a flat tummy right after I gave birth and then was walking on the treadmill two days after and lost all signs of having a baby within two weeks! I definitely recommend it if you have the energy!!!

Ya my dr also said as long as its not high impact it should be fine and he said even then I could do it but just to tone it down a little and stop before you feel exhausted and of course plenty of water before and after good luck, also walking is a great excercis eif you can make yourself do it ive have felt so nausated lately I havent been able to but hopely soon.