Yes, I can see why it sounds... Different
However, in my opinion, gender is a social construct. What you see in a scan are genitals, and whilst they will probably dictate the way the baby is brought up (clothes, pronouns, etc.) and define them as boy or girl for their whole life or for their first few years, they do not equate gender.
River could very well identify as a boy as she grows up.
This is my view on the subject and I wrote my comment according to it and life experience. I understand it is a very much debated subject and opinions differ on it.