This will sound like a cold answer, but its real
99% of the time women get pregnant when they want to.
I have been in a no condom, live in relationship since 2002. The girl in her 20s never accidentally got pregnant, married again in my 30s, the late 20s healthy wife never got accidentally pregnant.
But the one time I have a mistress , she accidentally gets pregnant
In 20 years of this hobby, I have never seen or heard of a story where a girl got knocked up while working. When they get knocked up, its usually by some guy they know and never one of us. Its like they know what day they can get pregnant, and act on it when they want it. I dont believe in accidents with women over 25