This is an interesting study: "...that recruited 782 young couples who did not use artificial contraception and who carefully recorded the day of every act (and there were a lot of them) until there had been 487 pregnancies."
The bottom line is that a single act of unprotected sex between a young couple has on average a ~1/20 chance of pregnancy.
http://www.bbc.com/future/story/20120313-sex-in-the-city-or-elsewhere
The bottom line is that a single act of unprotected sex between a young couple has on average a ~1/20 chance of pregnancy.
http://www.bbc.com/future/story/20120313-sex-in-the-city-or-elsewhere