Dictionary of Human Evolution and Biology

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First Trimester

First of the three trimesters of human gestation, consisting of months 1-3, and during which ovulation, fertilization, implantation, placentation, stage 1 of prenatal development, the critical period, and very early fetal events all occur. The first trimester starts with the onset of the last menstrual period before pregnancy, and is therefore a clinically, rather than a developmentally, defined interval.

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