I had those same problems after spending a few weeks confined to a hospital bed, under normal gravity. So what? Didn’t take very long to get back to walking and rebuild the muscles.
While lying in bed is the closest one can get on Earth to microgravity, it’s still not the same. Gravity does pull on your bones while you’re prone on Earth, nothing tugs at you in microgravity. Astronauts spending several months in space have reported the greatest difficulty upon their return to Earth. It is certainly possible to recover from this, but is something that has to be taken into consideration for long duration missions. Crewmembers will be weak as kittens near the end of their stay, and can’t maintain the same level of physical exertion in an emergency that they could at the beginning of the mission.
So, what about the baby born during it? 
I’m in suspense!
I have heard it claimed that gravity is required for proper development of an embryo. Apparantly there’s a key stage somewhere in between fertilization and implantation when the embryo goes from a homogenous mass of cells to a shape with a definite front/back/up/down polarization that requires a gravity field to occur. At least, that’s the speculation - I’m not aware of any experiments that would prove or disprove this.