The human gestation period of approximately 40 weeks between the time of the last menstrual cycle and delivery is traditionally divided into three periods of three months, or trimesters.

  • In the first trimester the embryo undergoes most of its early structural development. Most miscarriages occur during this period.
  • In the second trimester the embryo, now known as a fetus, is recognisable as human in form, but is not developed enough to be viable if born.
  • In the third trimester 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:

  • embryo to fetus: eight weeks
  • the edge of viability: 22 - 25 weeks (note the risk of death or disability, see the reference cited below).

Nevertheless, the concept of pregnancy as divided into three trimesters continues to be used in practical healthcare.

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