We are assuming that by "up the butt" you mean anal sex. If that is correct, then you ask an excellent question.
First of all, anal sex is the penetration of the anus by a penis (or finger or other stuff). There are a lot of nerve endings in and around the anus and many people find the stimulation of this area pleasurable. However, it can also be very painful - the anus is not as elastic as the vagina, and the anal sphincter responds to initial penetration by contracting. It is important to go slowly and wait until your partner is relaxed. You should also use lubrication such as saliva, vaginal secretions or a water based lube (water based will not deteriorate a latex condom). If your partner tries to penetrate you before you are ready, you risk tearing the anal tissue and bleeding which makes disease transmission more likely.
To become pregnant, a man's sperm needs to meet a woman's egg. The venue that this usually occurs in is the vagina - not the rectum. However, if the man does not wear a condom, or it slips off or it is worn incorrectly, there is a chance that some sperm may get into the vagina and a pregnancy could occur.
If you and your partner are going to engage in anal sex you should be aware that anal bacteria can cause serious vaginal infections and cystitis. It is imperative that your partner wears a condom and replaces the condom and/or washes his fingers thoroughly before he enters your vagina after anal intercourse. Also, if you use your tongue, cover the area with a dental dam or some kitchen plastic wrap or you run the risk of a stomach infection or a sexually transmitted disease (STD).
Anal sex is considered a high-risk behavior for HIV and other STD transmission. The delicate tissue in the rectum is prone to small tears that make entryway for the AIDS virus. Latex barriers (condoms, gloves) and lots of water-based lube, to ensure the condom doesn?t break, are essential.