Trisomy is a form of aneuploidy, an abnormality in which an organism has the wrong number of chromosomes. In humans, a normal baby will have 46 chromosomes in 23 pairs, with each parent contributing 23 chromosomes. When trisomy occurs, one of the chromosomes has an extra pair, resulting in 47 chromosomes instead of 46. The results of this extra data can vary, but tend to manifest in the form of birth defects, some of which can be quite severe.

The most common cause of trisomy is a problem in the duplication of chromosomes to create egg and sperm cells. Somewhere along the way, a chromosome duplicates itself twice, creating a full pair. When the egg or sperm cell joins with its counterpart, the extra chromosome is taken along, creating a set of three where where should be two. In some cases, a chromosome only duplicates partially, leading to partial trisomy.

Some of the more well known forms of trisomy are trisomies 13, 18, and 21. Trisomy 13 is also known as Patau syndrome, after the doctor who identified it. Patau syndrome is characterized by physical and mental defects, with heart defects being very common. Trisomy 18 is known as Edward's syndrome, and it is accompanied by severe mental and physical problems; most patients do not survive beyond a year. Trisomy 21 is Down's syndrome, a condition which is often accompanied by severe mental disabilities.

There are other forms of trisomy; trisomy 16, for example, usually triggers a miscarriage, while trisomy 9 leads to malformations in the skull and nervous system. The number in each trisomy refers to the chromosome in question; in some cases, patients have mosaic trisomy, meaning that the defect only shows up in certain cells. Trisomy 16 is believed to be the most common form of trisomy in humans, while trisomy 21 is the most common form in viable infants.

It is possible to test for trisomy, along with other potential birth defects, and women who are at risk for the condition may undergo prenatal testing to check for it. If a prenatal test such as an amniocentesis reveals a trisomy, a doctor will discuss options with the patient. In some cases, doctors may recommend that the pregnancy be terminated, because the baby is nonviable. Some parents may choose to carry on with the pregnancy anyway, for ethical or moral reasons. Some people with trisomy leave happy, full lives, while other infants survive for only a short time after birth.

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