Causes Fibroids Uterus

enlarged uterus during early pregnancy?
I have 7 weeks pregnant and had my first prenatal appointment. The doctor said my uterus is larger than it should and must have an ultrasound for next week. It seemed a little concerned and said it could be any number of things and do not be alarmed yet. I know exactly when my last period was (we all map) and I'm positive about how far along we are. We have no history of twins in the family either. The doctor did not agree with any reason for me why this might be a problem. Anyone have similar experiences or know of reasons that might cause? The only thing the doctor asked, unless he was sure how advanced it was if I never had fibroids, I did not. I prefer to know and be prepared to be surprised.
Believe twins should do to be "family" for them. He was completely natural twins (except that I was her husband) and was not in the family either. they need to start somewhere that is what the tech told me. Could it be that you have twins.
What are uterine fibroids?