What Is The Risk Of Pregnancy From Genital Rubbing Without Penetration Or Direct Contact With The Penis Head?

Hi I had a question regarding a possible pregnancy. Recently my girlfriend sat on top of me and began rubbing her vagina against the shaft of my penis. I made sure to cover the head so it didn’t touch there. She is not on birth control and I did not use a condom. I wanted to know how big the risks of her getting pregnant are ?