normal healthy sperms are motile (moves and swims )they can swim up the vagina if they are strong enough. even if you are still virgin(where the hemen is still unruptured or torn,remember that every full hemen in any virgin girl has a hole in the center opened so that blood comes out during period so if the sperms are in contact with your outer vagina the sperms can swim up in your vaginal canal reach the hemen enters the open hole go to your fallopians and fertilize an egg if you are in ovulation period )
however ,the chances are, of course, much less likely but it's still possible. the outside of your vagina to have contact with sperm, as some sperm may get inside, but the chances of it reaching an egg in that case are still slim. There are millions of sperm released with each ejaculation and less than one hundred will even make it to the egg, even after ejaculation inside the vagina. So you can see that contact on the outside would be VERY unlikely to lead to a pregnancy, but not impossible.
It only takes ONE sperm to be pregnant
Everyone ovulates at different times. And you can only get pregnant when you are ovulating, but remember sperm can survive inside the fallopian tubes for over 24 hours, so timing is everything.
the best way to protect from pregnancy using contraception methods or condom ,and if you are worried of being pregnant or not you can have a pregnancy home test from any pharmacy to insure that .
again ,it is possible but unlikely