How exactly do they determine the baby's gender at the 20 week u/s?

Greco1014

I've done some research on this, but I can't really find the answers I want. So maybe you ladies can help...

In about two weeks, we're going to find out our baby's gender at our next u/s. I can't wait! I'll be 20 weeks along by that time, so I'm expecting the gender info to be pretty accurate.

DH and I plan on sharing the baby's gender with anyone who'd like to know. I know my MIL is excited...she's already tried to buy baby clothes, but she couldn't find things that were neutral enough so she's waiting to find out what we're having.  

However, every time I mention to FIL that we're going to find out the baby's gender, he feels the need to remind me that they'll never know the gender for sure. He also tells me (over and over again) the story of my BIL's birth: apparently, they thought BIL was going to be a girl, the doctor said he would be a girl, and everyone was quite surprised when a boy popped out.

FIL is a sweet guy and I know he means well; he probably thinks he's giving me helpful advice. But by repeating this story so often, he's starting to stress me out. Yes, I know there's probably no way to be 100% sure of the gender before the baby's born, but I'd like to think the info we get would be pretty accurate. I don't have a preference where gender is concerned, but I don't want to spend the last half of the pregnancy preparing for one gender, only to find out we were wrong (it just seems like a lot of unnecessary work).

So anyway, I'm trying not to discuss this with FIL right now, but I know when the time comes to announce the baby's gender, he's going to keep telling the BIL birth story, and keep telling us that we can't be 100% certain about the baby's gender, and blah, blah blah... If I tell him we saw a clear pic of the baby's genitals, he's going to say that those can be interpreted wrong (he's already told me that, and he tends to repeat himself a lot).

So I'm wondering, is there any method used to determine the baby's gender other than simply looking at u/s pics? The next time FIL starts talking about uncertainty, I'd like to be able to politely say, "Well, of course we know that you can never know these things for sure, but our (insert technical jargon here) numbers were pretty high, and the doctor is confident that that indicates (insert gender here), so it makes sense to plan for that right now."   That might take care of the whole thing, and make me feel better, too.