If she is asking questions, she is ready to start learning...
First, however, ask her how SHE thinks it happens..... see what she is being told by her friends.
Or... ask her what she wants to know....
(This reminds me of the little boy that came home from school, and asked M.... "Where did I come from?" M. went into the big explanation...... and then, afterward, said... "Why did you want to know?" The little boy said, "Well, Johnny said he came from Cleveland!"
Those two questions are good lead-ins to finding out how much mis-information she has been told, or how much detail she wants.
ETA to add... to Busymom..... we had a 7th grader that was pregnant this year! So sad, isn't it? As far as abortion, I like what Dr. Laura said that her son Derek said, when she talked to him about abortion... his reply was.. "You mean they waste a baby?"