The most accessible explanation will be one that is just one level above the child's current level of understanding. This level can easily be determined by directly asking the child some simple questions about where babies come from.
Children can easily sense a parent's embarrassment or anxiety concerning this subject. To avoid this problem, it can be helpful to practice role-playing questions and explanations ahead of time with a partner or friend.
A common misconception among children is the "digestive fallacy," or the belief that the baby enters and exits "mommy's tummy" in the same way food does. Children need to be reassured that their anxieties about this belief are unfounded and they need simple, accurate information about sexual intercourse.
Often, children take such words as "egg" or "seed" too literally and create fanciful accounts of birth, using an agricultural or animal analogy. It can be helpful to address these misperceptions directly using simple pictures or models of male and female anatomy. Children may need to ask questions and hear explanations repeatedly until they can adequately process the new information.
At any age, it's important not to inundate a child with incomprehensible facts or to make a child feel stupid or foolish because she looks at reproduction in a fanciful way.