HM, I had a low lying placenta at my 18 week scan, but it was far enough away by the 33 week mark. However, my ob said that I could still have a vb if it was just 23mm away. My guess is that the birth centre might have tighter guidelines, so unfortunately, unless it moves enough (which is still possible) you might miss out. But the good news is that you should still be able to birth vaginally.