My personal opinion....It's a girl.
I was pregnant with 7 other girls, all due within a few months of each other. The only ones who were able to find out what they were having were the 4 of us that had boys. The 3 that had girls....The girls wouldn't stop moving or were "modest" and wouldn't show their goods on the ultrasounds and the mothers didn't know until they were born.
That theory again held up when my friend recently had her baby as well. It was a boy and she was able to find out no problems at her 18 week scan. I'm sure, had it been a girl, she wouldn't have known either until the birth.
:-d
to answer your question though, most women only have one routine ultrasound at around 18-20 weeks. Generally you don't get another one unless something is wrong. They're not going to do another just to try and find out the sex, sorry hun.