With unwavering exuberance, young Elizabeth takes readers month-by-month through her mom’s pregnancy, tracking her prospective sibling’s size from September’s “only as big as my bottom front tooth,” through February’s “as big as my stuffed rabbit,” and on to May’s delivery. Unusually for treatments of this topic, Elizabeth is very specific—about not only the baby’s physical development and Mom’s anatomy, but also about just how Dad’s sperm came to meet Mom’s egg. Thompson follows suit in cartoon-style watercolors, placing Mom and Dad under covers but interspersing views of the smiling family with lots of labeled inside views and enlargements. Confusingly, Elizabeth refers to the fetus as “he” throughout, even while relating how her parents are opting to wait to find out the baby’s sex—but for children seeking hard facts on the whole business, this makes a good alternative to the likes of Kes Gray’s Baby On Board! (2004), illustrated by Sarah Nayler, or Laurel Molk’s lyrical but oblique When You Were Just a Heartbeat (2004). (Picture book/nonfiction. 6-8)