Trimesters are the term used in pregnancy about the age of the fetus. Trimesters are made up of 3 months each. Thus, the first trimester refers to the first 3 months of pregnancy. It ends after the 3rd month.
13 weeks is considered the last week of the first trimester. This time marks a lessening of the symptoms of nausea and fatigue a woman may have experienced. The baby is roughly the size of a small apple.