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Well it all depends on how comfortable you are talking with her about it. My son who is almost 6 has came to me with some questions that I didn't think he would ask me ever, but instead of avoiding the question I made the choice to tell him the truth about certain things. You don't have to get detailed or anything, but a very broad answer will usually do the trick. This day in age our children are hearing and learning things at very young ages. We have to prepare them some how, I know personally I would rather have my son come to me and me tell him about this stuff then some kid at school who probably has no idea what they are talking about.
I think you should tell her. Otherwise, someone at school will and all she'll know is that you don't want to talk to her about sex (which could lead to very unfortunate consequences).
She could also easily google it. I'm not sure how careful you are about her time on the internet but it would take 2 seconds to look it up, and see most likley graphic pictures. I would explain it to her, use as little detail as possible. Let her have the information you want her too, rather than possibly the wrong info from google, or school mates.
are we talking about the pregnant 13 year old? by now, i'm sure you know about her posting a question on this site, using your account. talk to her openly about sex, and please give her the support she needs no matter what your family's decision is. you are in my prayers.