The second trimester of your pregnancy is an important development stage for your baby, and you may begin to notice more changes in your body. As the fetus grows, your abdomen will too, and you may need to put away your normal clothes in exchange for your first maternity outfits. During this time, you will hit a significant point in your pregnancy–the halfway mark.
The second trimester is the time from week 13 to week 27 in your gestational period. If you are counting by months, this will be months four to six. At week 20, you will reach the halfway point.