becky - it is the sperm that determines the sex of the baby, so i was worried that i would never have a girl due to my DH's family history (four boys in his family, no girls, and two grandchildren so far - both boys)... so i was very excited to find out last week that i am pregnant with a girl - woo hoo!!
good luck for your scan, and if this one is not a girl, then at least take comfort in the knowledge that it doesn't mean that your next child won't be! you are still capable of conceiving a girl!