Active mom question..?

I'm so confused and curious.. So the doctors say not to be lifting heavy things and all your second and third trimester.. But then I see all these work out mommies in their third trimesters and lifting heavy weights and such. When my MIL was pregnant with my SOs brother from lifting boxes and a mattress while moving houses, the baby pooed in her and swallowed it from the pressure and he almost didn't make it; so I'm extremely paranoid about lifting a finger lol. His mom is a crossfit woman; she's a total workout fanatic. She worked out regularly her whole life. She's super fit as well; I point this out because I know a lot of people are going to say if you've been doing it before pregnancy than its ok, but she still had difficulties.