TRIMESTER By N., Pam M.S. - April 29, 2013 noun. any of the three nearly 3-month periods into which a pregnancy is broken into. TRIMESTER: "Morning sickness usually dissipates for most women by the end of the first trimester." Related Psychology Terms EPSILON ALCOHOLISM DISTRIBUTED PRACTICE POLYDIPSIA CYCLIC ILLNESS 1 PUNCTUATED EQUILIBRIUM FARTLEK TRAINING PRENATAL COUNSELING MORNING SICKNESS ABORTION BRUCE EFFECT Cite this page: N., Pam M.S., "TRIMESTER," in PsychologyDictionary.org, April 29, 2013, https://psychologydictionary.org/trimester/ (accessed October 15, 2019).