It's an old wives tale. It says the faster the heart rate, girl, slower means boy. I've read about though and literature states there has been no research that has been able to back this theory up. My next door neighbor has 2 grown children, so when she was pregnant they didn't do u/s, only listened to the heartbeat. She said from the time they could detect her baby's heartbeat up until she was in the delivery room, the docs and nurses assured she would be delivering a girl! (Because baby's heart rate was so fast). Low and behold, she delivered a boy!
Re: first u/s pic