There is some fluidity in these hard cut offs. There are expected to be 40 total weeks in a pregnancy. The first trimester lasts approximately from weeks 1 to 12-14, second trimester from week 14 to 27/28, and third trimester from week 28 to delivery.
The fourth trimester refers to the 12-week period immediately after delivery. It consists of significant change as your baby adjusts to being outside the womb, and you adjust physically and emotionally to your new life as a mother.