TRIMESTER By N., Sam M.S. - 9 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 Dissociative Identity Disorder PMDD: Can It Be Cured? A Comparison of Maternal and Paternal Filicide PRENATAL DEVELOPMENT HOMOSEXUALITY (Female) DEMONOLOGY INFERTTLITY DREAM-STATE (D-STATE, REM- STATE) SEXUAL DEVIATIONS (GENERAL) READING DISABILITY (Paralexia, Dyslexia) Cite this page: N., Sam M.S., "TRIMESTER," in PsychologyDictionary.org, April 29, 2013, https://psychologydictionary.org/trimester/ (accessed August 8, 2022).