did I make a selfish decision?

Alex • Blessed to be mom to our miracle IUGR baby boy! I've never met someone as strong and brave as he is!
We decided to go ahead to do a private ultrasound to find out the sex of our baby early (17 weeks as opposed to 20). We still are having our 20 week anatomy scan ultrasound at the doctors though because it's routine. Our doctor had to okay the 17 week ultrasound and it was done by a medically licensed sonographer. My mom tells me "you sure that might of been bad. Too many ultrasounds will hurt the baby." By his next ultrasound he will have 4. Is she right? Did I make a terrible decision by finding out early? Am I just selfish?