From medical physics perspective,I don't think that will matter. One it is for your LP's teeth, far from where your baby is (I also doubt how much dose you would really get from that); two she is already 28 weeks.The most sensitive stage is first trimester, especially the first 8 weeks or so. But of course if we can avoid, we do not want any type of x-ray whatsoever. Just my 2 cents Disclaimer: Not for any medical advise. Please listen to your OB :)