For the term trimester used in academic settings, see Academic term
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.
This is the time where your fetus is most at risk for exposure to toxic substances that may impede development or cause birth defects in your baby, so it is very important you seek healthcare advice as soon as you discover you are pregnant.
The firsttrimester begins from the end of your last menstrual cycle.
During the firsttrimester you should decide whether you want to see an obstetrician/gynecologist or traditional medical doctor or a nurse midwife to deliver your body and care for you during your pregnancy.
It is during this firsttrimester that the fetus is most susceptible to damage from substances such as alcohol, drugs, certain medications, and illnesses such as rubella (German measles).
During the firsttrimester, both the mother's body and the fetus are changing rapidly.
The embryo develops rapidly and by the end of the firsttrimester it becomes a fetus that is fully formed, weighing approximately 1/2 to one ounce and measuring, on average, three to four inches in length.
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