1) Your hips may suddenly get wider way before your belly even starts to grow, so you think you've got time before you have to break out the muu-muus and the wide load pants, but nooo...
2) You can't sleep comfortably on your back for most of the pregnancy. I honestly did not know this until I tried it and felt like my blood vessels were being impeded like a hose with a kink in it.
3) I didn't know I'd have to prop up my belly when lying on my side, so it wouldn't hang painfully towards the bed like an enormous third breast. I love my maternity pillow.
4) That I'd get heartburn that would cause me to choke on my own vomit quite spontaneously in the middle of the night. (It happened so often I started to refer to the experiences as my "Janice Joplin Moments". Insensitive? Yes, accurate description, yes as well.)
5) I would never (without being under the influence of an extremely mentally debilitating disease or some really bad acid) have imagined in a billion years that the nurses would have the gall to ask me 15 pages' worth of questions while I'm lying on the hospital bed undergoing contractions every 2.5 minutes. Bear in mind, when I say 2.5 minutes apart, that is measured from the start of one contraction to the start of the next... Not giving me much of a reprieve in between.
6) I had no idea I'd produce "enough milk for a small nation" as the infant nutrition specialist put it. This means that when my children suddenly decide to pull away, I then spray my poor unsuspecting offspring in the face with milk. And my arm. And my t-shirt. And my bra. And my sofa.
7) No one told me that drinking really cold water could make me spontaneously start lactating. I knew to expect it when hearing a child cry, but even looking at my baby can do it to me too. My husband says he wishes he had that effect on me. I'm glad he doesn't. My clothes are soggy enough. Leaning forward makes me lactate too.
8) No one thought to mention projectile poops. My child's not my own. Nuff said.
9) Not one helpful soul warned me that I'd have the longest period of my life after having a baby and would, at first, require pads the size of place mats. Not one. I'm just sayin'.
10) What no one could ever warn me about is how enormously overwhelming my feelings are for my little ones. I'd crawl through hot coals, broken glass, and a Hansen concert for my babies. God help their future suitors.
EDIT: I must also add that NO ONE ever mentioned mastitis. Freaking OW. It's like having a toothache in your boob.