The day my daughter turned 10 I talked to her about the birds and the bees in detail. However, two years earlier, I sat down with my two daughters and explained to them, in detail, what was going on in Mommy's belly and how their little brother was being made. Each month, we discussed what was happening to me and to the baby. I used pictures, diagrams, my ultrasounds, and explanations to make my girls understand everything I possibly could about pregnancy.
Your child is different than mine. My first daughter was ready to know about her body, sex and relationships. Yours might not be.
Your family dynamics are different than mine. My children's father and I are divorced. I had my third child on my own without any one with me. My children were exposed to a different type of environment than others might be.
You will know when you need to talk to your child. Do not wait too long or you may end up explaining how the baby is growing, but not inside yourself.