Do they really know ?

I heard that when they test your blood's for chromosomes at 12 weeks they find out the sex of the baby but don't typically tell you in the UK untill it is confirmed by ultrasound ... am I right ? I was in hospital yesterday and she went to check my blood results as I wasn't due to see my midwife untill today , but she referred to baby as she after that then said oops I'm a bit optimistic ... so I'm wondering did she know or was she just being optimistic ? lol !