Hi.
In order for pregnancy to occur, semen (which contains sperm) must reach the vagina. During unprotected anal sex, it's certainly possible for semen to leak out of the anus and possibly travel up the vagina. Although this is only a slim possibility, stranger things have happened.
However, this leads me to the following point: if you're having penis-anus sex without a condom, not only are you risking pregnancy, but also many different kinds of infections. Dr. J
You have more to worry about than just a baby. Anal sex is dangerous for you because it is ripping your rectum muscles and the longer you do this it will eventually cause trauma to your internal body and you can have chronic lose of control of bowel and other disfunctional problems> Be Aware.. Look up the Cause and Effects long term. Your boyfriend is enjoying himself but he can get a very serious illnesses and bacteria from the rectal sex..
Sorry to be the barrier of bad news but you are to young to be having health problems later on from his urges!! You have a child to think of, Get on Birthcontrol.
There is always a risk of pregnancy if his penis is anywhere near your vagina, some people are unlucky enough to fall pregnant without actually having vaginal intercourse.
If you do not want to get pregnant then I would highly recommend using a form of birth control, there are so many choices out there that there is no reason not to nowadays.
Take care