With my son, my OB told me not to lift as they were worried about my cervix, supposedly lifting heavy objects puts pressure on everything down there and can cause your cervix to shorten (which can lead to premature birth).

This pregnancy my cervix is doing fine but once again I have been told not to lift heavy objects and not to lift my 3 year old as a precaution, I am not even allowed to carry in the shopping.

I am sure this isnt the same for everyone though!