I’m no expert on this subject, but I have been pregnant once (so far) and I did my best to stay fit while I was blowing up! I think there are two main things that having a fit pregnancy depend on...
How fit you were before you got pregnant
What kind of pregnancy you have
So, #1 – Having a fit pregnancy depends on how fit you were before you got pregnant. When I got pregnant, I was in the middle of training for a duathlon (run 2 - bike 12 - run 2), which I did when I was 2 1/2 months prego.
Personally, I think I was pretty fit to begin with. I kept to my training schedule (for the most part), but didn’t train as hard as I would have had I not been pregnant.
And #2 – Having a fit pregnancy depends on what kind of pregnancy you have. Some women get morning sickness, some women are put on bed rest...in my case, the worst thing that hit me was heartburn.
So throughout my pregnancy, I was given the OK from my doc to do what I had been doing before I was pregnant. That made being able to stay fit while pregnant easier for me.