I was in your exact boat when I got pregnant again. Just had a misscarage and was worried. I started yoga right away.just start slow and dont push it. There are lots if YouTube videos to help you know what poses are safe. If you didnt exercise before yoga is still safe as long as you are careful with the poses. My mother in law did yoga religiously and her labor was 10 mins once she got to the hospital. It really helps me with the back pain now. Don't know what I would do without it.
The best time to exercise is now. I exercised all the way through my pregnancy. Im 30 weeks and I have missed one day due to being tired. If you were already active prior to conceiving then work at that same level. But dont introduce any new workout activity