Kids books on pregnancy?

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I am currently pregnant with my third.
I have a 4 and 6 year old who keep asking alot of questions in regards to how babies are made, what they are doing in my tummy e.t.c.
I thought it would be a nice idea to get them a book about babies, pregnancy and how babies are made.
Does anyone have any ideas on good books in relation to this subject for kids?

I bought Mummy Laid An Egg by Babette Cole.
It gave dh and I the best laugh we've had in ages, sadly I haven't been brave enough to give it to the dc's :o

Bump -children same age, would love some recommendations! We have Babette Cole, but wondered about an alternative/ additional one? (Didn't think I was prudish, but thought of 4 yr old's questions about positions makes me feel

Mummy laid an egg for my DS who is now 6 but gave it to him last year when my sister got pg and he wanted to know how the baby got there!! I completely agree with sydenhamhiller though about positions!! I kind of skipped thru that and turned two pages together!! We had a few days of lots of questions about the book, which we answered as honestly and accuratly as possible and relevantly for his age, but when we told him earlier this year that he was going to be a big brother no questions about how the baby got there or anything. i think it all depends on which route you want to take, a humourous, light hearted but informative, of factual, Usbourne books do a variety of good sex educatrion books. Hope that helps!!