Yep like the others said it depends on the baby on the day. I told my sonographer before the scan that we wanted to find out the sex and she said she will try but it depends whether bubs is in the right position. Lucky for us he was and did a nice big leg spread and lets just say, its definately a boy!! I know some people who have had scans and bubs was lying the wrong way, they told them to go for a walk and come back and theyll try again.
I had mine at 18 weeks 6 days because my 19 week mark was around Good Friday and they were booked out during school holidays.