Adriana,
Don't worry about a thing! The ultrasound is just to see how big the baby is (approximately) and its position. I measured several weeks larger than normal and since both my husband and I are tall there was speculation about us having a very big baby. The ultrasounds estimated he would be between 8 and 9 pounds, still very much in the normal range. When he was born he was actually 7 lbs 8 oz.
Ultrasounds are strange things. The margin of error for calculating weight is 1 lb, which is quite a lot in a newborn. When had our ultrsound they detected a shadow on the baby's left shoulder or neck. Of course, I totally freaked out. The doctors didn't do much to ease my worrying either, although they weren't mean, they were just all business. I was sent to a specialist at Hackensack Hospital who did not detect anything but wouldn't say there was nothing there. Then I went to a specialist in Manhattan who explained that ultrasounds are shadows and he didn't detect anything either. He put my mind at rest by saying that I would have a completely normal baby. My doctor recommended a C-Section just to be on the safe side and my completely normal, 7 1/2 lb baby boy was born 10 days before my due date.
So the moral of the story is, your baby is just fine. He or she is either a big healthy baby or a normal baby in a sideways position.
Good Luck.