Request an ultrasound?

So I was wanting to get an ultrasound to see if I could find out the gender BEFORE Valentine's Day to suprise my hubby but my midwife said my next ultrasound will be 2 weeks after Valentine's Day when I'm 20 weeks, just wondering how others went about organising their own ultrasound and if I'm needing to see my midwife about it or if I can do it without her? Thanks