low lying placenta

With low lying placenta at 20 weeks, will it gradually move up? Gynae just said i just shouldnt exert. But i can carry on walking but my journey to work n all is quite a distance. Gynae doesnt seem concern about it. And if it doesnt move up, what happens? Will it cause miscarriage or anything? So afraid cos its my first bb. Pls advice. TIA! =)