The first trimester of a pregnancy is the first three months of the pregnancy. The second trimester is the period from three months to six months, and the third trimester 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.
...the proposed school's trimester calendar.
trimester in British English
(traɪˈmɛstə)
noun
1.
a period of three months
2.
(in some US and Canadian universities or schools) any of the three academic sessions
Derived forms
trimestral (triˈmestral) or trimestrial (triˈmestrial)
adjective
Word origin
C19: from French trimestre, from Latin trimēstris of three months, from tri- + mēnsis month
trimester in American English
(traɪˈmɛstər; ˈtraɪˌmɛstər)
noun
1.
a period or term of three months; specif., any of the three three-month periods of a human pregnancy
2.
in some colleges and universities, any of the three periods into which the academic year is divided
Word origin
Fr trimestre < L trimestris, of three months < tri-, three + mensis, month: see moon
Examples of 'trimester' in a sentence
trimester
And I would say, small as you still are, you are well into the third trimester.
Salley Vickers INSTANCES OF THE NUMBER 3 (2002)
As summer approaches and the last trimester of my mother's pregnancy, she becomes obsessed, apparently, with polio.