Help!!! Doctor saw twins

I had my first ultrasound today. And they told me I'm 5w3days... First she did a transvaginal ultrasound and my doctor told me there were two sacs (twins) I got excited. But then she did a abdominal ultrasound and only saw one sac.. So I'm confused which one is more accurate?? I go back in two weeks just to make sure if is one or two.. Any suggestions of what's going on?? Or has it happened to any of you ladies??