This book, It's So Amazing, is so amazing!
It says the book is for ages seven and older, and has sister books for younger and older kids. I read it to my older son when I was pregnant beginning when he was just under two and a half years old. My younger son is now three years and three months old, and he is interested and engaged in the book (which is not the case for every book, as it was for my older son at this age). I chose this book instead of the book for younger kids because I wanted more detail to answer their questions. I also use the drawings as a jumping point for me to explain things, rather than reading all of the words.
This is a great book because it talks about how boys and girls are different, and what our body parts are called, and how they change throughout our lives. I think it is important to give kids the words for the things they notice and are curious about and to normalize them.
It also discusses love and sex, and what eggs and sperm are, and what happens as and after they meet. The kids love the real-size images of a fetus and fetus to-be. The page with the actual birth description has nice images for explaining how a baby comes out of a mommy. It even touches on the different gestation periods of different mammals, multiple births, adoptions, preemies, and "not-nice" touches.
Both boys, 3 and 6, are interested in this book. The younger one thinks it is interesting that boys won't have babies inside them, and that they will always have a penis, and never a vagina. The placenta, umbilical cord, and birth process are also interesting. He wanted to see his baby pictures after reading it, then his brother's, then mine and his daddy's.
My older son likes the differences between the gestation periods of different animals, and loves the page with the fetus (and fetus to-be) at different stages and sizes. Both enjoy learning that they will hit puberty someday, and seeing how their bodies will change.
This is an excellent staple for a young child's library. It has given us a great framework for many discussions and helps the boys understand human bodies a little better.