What's an omega girl? And also can male omegas impregnate themselves?

I just started reading this manga and it says that in elementary school the whole class went for a gender check to see what they are and one of the girls found out she was an omega and the whole class outed her and made fun of her but I am so confused like.........????? doesn't that just make her a girl???? And if a male omega has a womb and sperm can't he get himself pregnant or clone himself lol. Someone please make sense of this to me because I want to understand it but it's just so confusing.