I can't believe we're still answering this question, time after time after time!!!
Girls anus and her vagina do not have any connection. If you ejaculate your semen in her anus, unless she makes a really determined effort to scoop it out and insert it into her vagina, in which case, she deserves everything that happens, you cannot get her pregnant from anal sex.
If the two of you have enjoyed anal sex, when you withdraw your penis from her, don't allow your semen to run out, down across the perineum, and across the lips of a vulva. I assume you are having sex in the doggy position, with her kneeling on her hands and knees. As soon as you withdraw, just tell her to turn over and lay on her back. That way there is not even the remotest chance, which would be incredibly small anyway, of your semen impregnating her.
Even if your semen did run down and trickle across the lips of a vulva, a rundown to clitoris, it takes considerable effort for any of that semen to make it to the top end of her vagina, where it could enter her worm through the neck of her cervix. Believe me, most couples can try for weeks and weeks or months, sometimes even years, making a determined effort to get the girl pregnant, without any positive result, so the chances that one random sperm could make it from the outer lips of the labia, all the way up her vagina, through the neck of a cervix, into her worm, then up one of the fallopian tubes to fertilise her eggs, while there is the remotest possibility, but it is so infinitesimally small, that you really don't have to worry. If you come in her vagina on the other hand, your semen will be deposited right at the entrance to her cervix, and you increase the chances by an order of magnitude, so if you want vaginal sex, where a condom, every time!!!
If the two of you are having sex regularly, you should really be discussing proper contraceptive measures. She could have a contraceptive implant, she could have an intrauterine device fitted, or she could take the daily mini pill. These days, men can also have a contraceptive implant which means that they cannot make a girl pregnant. It would make a change to see some of you guys taking responsibility, but it's not usually the case. You usually wait for us to do the sensible things in life, while you want to have your fun. Still, some guys do have a sense of responsibility, and I applaud them.
Now, having answered this question for the millionth time, I live in forlorn hope that we won't see asked again.