I agree with everyone else about not tiptoeing around this - no way would I avoid books or pages about this! Your DS might find Mummy Laid An Egg too babyish, but if not, I recommend it. Actually this is one of the many areas where David Attenborough Life series DVDs have been very helpful - having DS know about how sexual reproduction evolved and e.g. about how scorpions do it is useful background for talking about humans. Life on Earth and Trials of Life are both very good for this (and lots else).
