Its just some trusts policy. She just gets to look forward to the surprise, surely its a nice thing to look forward too. Its never a definite anyway.
I didnt kknow for my first and didnt want to know for the second but we all noticed it was a boy. The other mums would find out if your daughter told anyone what she was expecting.
Beryl is right if she really wants to know she could get a private scan. My mum (who is a midwife) reckons you get 3 the early one that scans for downs, a first trimester dating scan, a second trimester scan to check for other abnormalities and sexing for around 200 quid.