Needing to pee. A lot.

At some stage your bladder will contract to approximately the same size as a walnut. Once this point is reached, you won't be able to go more than an hour or two between trips to the bathroom, and this includes during the night. I think it is some kind of plot by nature to get you used to waking up a lot for a hungry baby. Closer to home, the size of your bladder is a delight to ultrasound technicians, who will always require you to drive over a very bumpy road with a full bladder to get to their appointments.