Did you ever watch Dragonball Z? All the scenes where Goku or Gohan are training to get stronger, building their bodies to be powerful? Piccolo wearing insanely heavy gear to improve his speed?
You don't need to do sports to train your body. You are giving your body the opportunity to learn how to be strong, to learn endurance. You can't do that without practicing those things, using your strength, pushing your endurance.
That said, I have found working with kids to be very helpful. I work with a particular young kid who absolutely loves being lifted and thrown about. I use him as my weight training and it is a whole lot of fun, not to mention rewarding making his day more fun. Maybe there is something you can find that is similar?
In biology we often talk about how evolution isn't about perfection, it is all about just good enough. You don't have to be the fastest runner, you just have to be fast enough to survive. You don't have to be the strongest, just strong enough. And when it comes to surviving it is not on an individual basis, it is about the genes surviving, so if only two out of 3 women survive childbirth but those survivors have more than 3 kids on average that is good enough.
Pregnancy is one of the most dangerous things a person can do in modern society and the dangers are downplayed to an insane degree. If we truly valued it then we would thank the people who carry babies to term by making sure they are supported and respected, along with turning research towards promoting health and wellbeing. Our choices display what we value, and in this society we don't value pregnancy, we appear to distain it.