One thing you can do is wear elastic waistband pants.
Another is drawstring scrub pants.
Really low, hipster regular pants will still work.
Or you can take a ponytail hair tie and anchor it to your button, pull it through the button hole and fasten over to the button again. You do have to wear a long enough shirt to cover your entire zipper for this often makes it easier for your zipper to come down.
Or the maternity pants that are low waisted elastic all the way around will fit this early in pregnancy.