Congratulations, rvelvetcupcake!
I took the MCAT for the first time while 7mo pregnant, and I still have mixed feelings about it. On the one hand, even though I didn't do nearly as well as I hoped, the experience of testing in the actual center was tremendously different even than the online practice tests and having that experience was valuable. I also (eventually) came away from it with a feeling of, "Well, if I can pull off passable under lousy circumstances, then I bet I can take this sucker again and do better." (I was also one month into a new job and dealing with truly awful weather conditions in the week leading up to the test).
That said, restless nights (because your hips and back just plain hurt there toward the end) and difficulty concentrating take their toll, and depending on your testing site, those necessary bathroom breaks and snack breaks eat into your zen time between sections.
All else being equal - that is, if I had been ready at an earlier time - I would have chosen to take it while less pregnant. Morning sickness can be bad up to 15wks or so, but you throw up and then you go back to work. In many other ways, you'll probably feel great. Later on, the physical strain is more constant. Either choice is still better than trying to take it after the baby arrives. It can take time to recover physically - maybe more than you're banking on. You're not going to get any sleep for a while and the emotional roller coaster is also rough on concentration. That beautiful little creature that is living right up under your heart will be 18 inches further away and for me, even though it was my second child and I knew the drill, that was reason enough to cry like my heart was breaking.
The most powerful thing about taking the MCAT earlier is knowing that if everything crashes and burns the first time around, you can take it again, with better perspective. I did, though I had to wait a year - and I did do better. Whatever your choice and whatever the outcome, be determined to keep trying.