Definition of Trimester

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Trimester: The nine months of pregnancy is traditionally divided into three trimesters: distinct periods of roughly three months in which different phases of fetal development take place.

The first trimester is a time of basic cell differentiation. It is said to truly end at the mother's first perception of fetal movement (quickening) which usually occurs around the end of the third month.

The second trimester is a period of rapid growth and maturation of body systems. A second-trimester fetus born prematurely may be viable, given the best hospital care possible.

The third trimester marks the final stage of fetal growth, in which systems are completed, fat accumulates under the soon- to-be-born baby's skin, and the fetus at last moves into position for birth. This trimester ends, of course, with the birth itself.

The word "trimester" came into English in 1821 from the French "trimestre." It, in turn, evolved from the Latin "trimestris" -- from "tri-," three + "mensis," month. Trimester is thus related in origin to menses and menstruation.

(In the UK, trimester is always a period in pregnancy. In the US, trimester also refers to a three-month period into which the school year may be divided.)