Worried!

Vikki
Im a bit nurvous i have a scan monday i will be 21 weeks. I had a scan at 17 weeks a private one they told me my baby is a little girl! I keep having bad dreams of the scan and they tell me im having a boy and i have to return all of the pink bits family members have bought! I know it sounds silly i have 3 handsome boys already so i was so so excited when they said baby number 4 was a girl... how accurate is a gender scan at 17 weeks?