It can also increase the risk of STDs, so no harm in getting pregnant, but possible harm in getting HIV/hepatitis/chlamydia/syphilis.
But, as others have said, it can be made safer. Or not, if by ignorant and inexperienced boys/men.
All that can be said of unprotected penis in vagina sex though. The increased risk obviously comes from possible tears whilst not using condoms and lube, however these things can also be the case when inexperienced people have PiV sex.