The firsttrimester of a pregnancy is the first three months of the pregnancy. The secondtrimester is the period from three months to six months, and the thirdtrimester is the period from six months to nine months.
At the end of the first trimester, the fetus is about three inches long.
2. countable noun
In colleges and universities in some countries, a trimester is one of the three main periods into which the year is divided.