Actually, we'd very probably say pants. Well we might perhaps say knickers. But pants is the more standard word. For men it would be underpants (which means briefs or boxers or trunks or Y-fronts), I have no idea why it is not underpants for women as well. But it isn't.
Underwear just means whatever you are wearing underneath your clothes - bra and pants, maybe a vest as well if it is cold, or thermal leggings or even a longsleeved thermal top. All that is underwear. So although pants/knickers are included in underwear, underwear does not just mean knickers. (Or pants.)
And talking about pants - that reminds me. Some years ago now, an aunt of mine (who was American) was talking to me about an event that I was planning to attend, she was interested in what I was thinking of wearing.
What she said was - try to imagine how just surprised I was to hear this very elderly, very elegant, and very sophisticated lady say such a thing - what she said actually was, "Are you going to wear pants?"
It definitely did take me a moment to realise that she was asking if I was going to wear trousers.
![Laughing :lol:](./images/smilies/icon_lol.gif)