Can a girl get pregnant if the guy is naked but she is wearing underwear? Please Help!?

Just recently me and my girlfriend where getting in the moment, she proceeded to take off my underwear, We continued to dry hump i guess you can say, and finished, I did not ejaculate, but there was some but very small pre ejaculation (I'm not a hundred percent sure if it even got on her underwear, it felt like it didn't) I was wondering can this impregnate a woman? Can pre ejaculating go through a woman's underwear? And how long can sperm survive outside a woman's underwear? Please help!