For a start, unless you're paying for one which is specifically for that purpose, it's not. It's a medical scan to check organs etc.
But it's also not a, "gender" scan. All it will tell you is whether your baby has a penis or a vagina. That's it. It'll tell you that they're biologically male or female (intersexuality, androgen insensitivity syndrome etc aside). The inaccuracy of it irritates me. That penis or vagina will probably determine their position in the gender hierarchy, but it's still all we know about them at that point.
I'm a feminist and believe gender is socially constructed, with girls and women at the bottom. So I get really irritated by, "team pink!", "gender reveals". I know it can help us to bond with our bumps, but they're not even born yet and we're already basing our preconceived ideas of who they might be around their gender.
I don't begrudge anyone this, it just annoys me. AIBU?