That the sleep deprivation can be worse than anything you've already experienced.
That the hormones flooding through you make you cry uncontrollably and make you feel like the worst mum in the world ever.
That you'll get some lovely midwives and some real monsters in the hospital who after your csection try to help get bf established by ramming your udders into your baby's mouth, then alternately tell you you'll have to express as you're disturbing everyone else on the ward with your bub's crying.
That "real" nappies are a fantastic idea, look lovely, feel nice etc but are totally impractical when you're so knackered you don't have the energy to change them every 2 minutes when your LO has a wee/poo/both.
That meconium has to be experienced to be believed, and that you can only clean it off with wipes which you have to hide from the m/wives who expect you to use dinky bits of cotton wool and water.
And of course, that just because you have a great baby who wants to sleep all the time, you shouldn't let them....and you have to wake them every couple of hours to make sure they get enough milk or they end up with jaundice (which you naively think is a nice sun tan)...
That hypnobirthing may not get you through the actual birth, but sure as hell helps prevent you panicking throughout the pregnancy.
That your pregnancy can be completely plain sailing up until you hit 40 wks then your blood pressure can screw up all your plans.
Ooh, sorry for sounding a bit bitter, still getting over it 14wks later!