Why tri?

Yesterday I got to wondering, why is pregnancy divided into three trimesters? It would make more sense to me to divide 40 weeks of pregnancy into four quarters. Plus it would be easier for my sports-minded husband to follow, if he could track my pregnancy like a football game. Am I the only crazy out there that gets irritated when trying to figure out what trimester I'm in between week 13-15 and week 26 to 29?