Small head size?!!!

I'm 32 weeks today with twin girls. Yesterday I had a regular biophysical profile and Baby As head is measuring small compared to her body. I'm freaking out about it. My dr didn't seem to be worried. He said he wasn't sure how accurate of a measurement it was because of her position and we're going to measure again in 2 weeks. But I can't help but stress about it. I made the mistake of googling it and now I'm freaking out even more because it could be due to mental defects. Has anyone else experienced this? 😖😩😫