I count the weeks, too. One book tells me I was in the 3rd tri at 25 weeks, another says I won't hit it till week 27 or 28!
It's so crazy to think about actually going early. I guess I always went on this assumption that generally a first time mom will go "over" the projected due date. It would be strange to suddenly be having my baby and still have a week or 2 until my maternity leave starts. I set my maternity leave to start on the week of my due date- especially because (without a doctor's note, which hopefully I will have from my midwife) I only get 6 weeks paid, and had to plan around that.