Erika, like you I stopped running in the first trimester due to morning sickness. I ran a marathon at 4 weeks and was absolutely fine, but by 6 weeks I stopped running completely. I was just feeling too nauseous and sick. This got better around week 16 but I didn't run again until 18 weeks. I only did small distances: 2.5 to 3 miles, but it was a start. The long break meant I was a bit out of shape but it didn't cause me any problems. I'm now up to 6.5 mile runs (at 21 weeks) and I'll probably stay at that distance. I'd say go slow, run on the flat and listen to your body. If it doesn't feel right then stop, your body will soon tell you if it doesn't like what you are doing.