Safe to exercise pregnant?

I'm only 12 weeks 4 days. I'm not athletic and don't work out on the regular, once in awhile I'll go into workout phase and my weight has stayed pretty consistent for years. But right now I'm the heaviest I've ever been and I don't want to have a hard time getting my pre baby weight back after I give birth. I just want to make sure I stay active and not gain a lot of unnecessary weight. I don't want to do ab workout or lift weights, but I'm just thinking about walks outside or on the treadmill. That should be safe right?