This is so incredibly common. They only bother checking for it at the 20 week scan because for some women this will be the only scan they have...but it's such an inconsequential finding at 20 weeks that they really shouldn't bother.

For the vast majority of women by 34 weeks it will not be an issue.

Imagine your uterus as a blown up balloon, with the tie at the bottom being your cervix. Draw an "x" down near the bottom of the balloon, which is the edge of your placenta. As the balloon is blown up more (ie as your baby grows), the "x" moves further away from where the tie is. This is what happens to your placenta as your uterus gets bigger. The stretching pulls it further away from the opening of your cervix.

Don't stress, I'd nearly bet my first born that it won't be a problem for you later