I'm an ultrasound technician and I often lie to parents about the gender of their child
Why would you lie to parents about their child's gender?
It's just that I don't like the fact that they will be so disappointed to discover their child is a boy after delivery.
Lying to patients about something medical related is unethical medical practice. Be honest with them before you find yourself in a lawsuit about the matter and your license taken away.
I just don't tell them the truth because they are already so stressed by the pregnancy.
That's not for you to decide. It's possible that you'll lose your license if you continue to lie.