Hidden twin?

Lauren
So we went for our first US today. I am 6w5d but we had fertility issues so we get to scan early. At the REs office today, he said that we may have twins because the fetus is so low that it could be hiding another one. He wants us to come back in a week to check for sure and see. We only heard one heartbeat but since it is so early unless he sees it on the monitor he can't gauge it. Has anyone else been told this? Anyone have results? Thanks!