I NEED HELP The trimesters of fetal growth: calculate the percent growth between each of the trimesters. You should have three calculations: percent growth between the first and second trimester, percent growth between the second and third trimester, and percent growth between the first and third trimester.. To figure percentage, subtract the length of trimester 1 from trimester 2, then divide this amount by the measurement of trimester 1. Repeat this process for each time period requested. The first calculation is done for you here. Example: trimester 1 = 3 inches, trimester 2 = 14 inches. 14 in - 3 in = 11 in. 11 in /3 in = 3.67 and 3.67 x 100 = 367% increase in size.