Trimester

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It has been suggested that this article or section be merged into Pregnancy#Timeline of a typical pregnancy. (Discuss)

For the term trimester used in academic settings, see Academic term

Trimesters divide the human gestation period into three periods of three months. The entire gestation period lasts for a duration of approximately 40 weeks, between the time of the last menstrual cycle and delivery

The first trimester is the period of time from the first day of the last menstrual period through 12 weeks of gestation. It is during this period that the embryo undergoes most of its early structural development. Most miscarriages occur during this period.
The second trimester is the period of time extending from the 13th to the 27th week of gestation. During this period the embryo, now known as a fetus, is recognizable as human in form, but is not developed enough to be viable if born.
The third trimester is the period of time extending from the 28th week of gestation to delivery. It is during this period that the fetus reaches viability, and may survive if born prematurely.

The use of modern medical intensive care technology has greatly increased the probability of premature babies living, and has pushed back the boundary of viability to much earlier dates than would be possible without intensive medical assistance. In spite of these developments, premature birth remains a major threat to the fetus, and may result in ill-health in later life, even if the fetus survives the birth and subsequent intensive care.

The actual boundaries of when an embryo is regarded as a fetus or a fetus becomes regarded as potentially viable depend on the definitions of these terms, and do not necessarily fit neatly on the classic trimester boundaries. Note also that these boundaries are the matter of both medical and political controversy.

Some timings that have been used are: