Ind. Code § 16-18-2-355

Sec. 355. "Trimester", for purposes of IC 16-34, means any one (1) of three (3) equal periods of time of normal gestation period of a pregnant woman derived by dividing the period of gestation into three (3) equal parts of three (3) months each and to be designated as the first trimester, second trimester, and the third trimester, respectively.