Most of the time, when people are squabbling online about fitness during pregnancy, they fall into one of two camps:
EITHER: "You should do nothing more strenuous than walking and yoga, and you should lift nothing heavier than a 3lb dumbell"
OR "You can do anything you did before pregnancy - run marathons, lift your 1 rep max deadlift, intense cardio - you're a badass mama!"
While the debates often focus around who's wrong and who's right, I think they both kinda miss the point. It's just not really black and white like that and the truth lies somewhere in the grey area in between (like most things in life).
Exercise is extremely important for the health of both the mother and the baby,
If we avoid the very extreme forms of exercise that really raise the heart rate or body temperature of the mother, and avoid any knocks and falls, exercise is very beneficial to the growing baby.