Chromosomal Abnormality

Definition - What does Chromosomal Abnormality mean?

A chromosomal abnormality is when a person, embryo, or fetus is missing a chromosome, has an extra chromosome, or has an abnormal section of a chromosome. These abnormalities are linked to recurrent miscarriages, failed IVF attempts, unexplained infertility, and many genetic disorders. There are many screening tests that can be done to detect chromosomal abnormalities.

FertilitySmarts explains Chromosomal Abnormality

Chromosomal abnormalities fall into two categories: having the wrong number of chromosomes (aneuploidy) and having abnormal chromosome structure. Aneuploidy could be having too many chromosomes (such as in Down Syndrome, where the individual has an extra copy of chromosome 21 in each cell) or missing a chromosome (such as in Turner Syndrome, where the individual is missing one X chromosome). Sperm can be the cause of the irregular number of chromosomes. Some lifestyle choices, such as tobacco smoke and exposure to certain chemicals, can cause DNA damage in sperm and result in aneuploidy of an embryo. The age of the mother is another important factor in aneuploidy. As a woman ages, the ability of her egg cells to under cell division decreases, and mistakes become more common. This leads to an increased chance of producing an egg with an abnormal chromosome number.

Abnormalities can also occur if the chromosome has an abnormal structure. These can be from the deletion of sections of chromosomes (such as in Cri-du-Chat Syndrome, where the individual is missing a portion of chromosome 5), or having extra sections of chromosomes (such as in Pallister Killian Syndrome, where parts of chromosome 12 are duplicated).

Over half of early-pregnancy miscarriages are caused by chromosomal abnormalities. Abnormalities occur in 0.5-1% of all live births and can be detected using several techniques. Amniocentesis takes a small sample of the amniotic fluid surrounding the fetus using a needle through the abdomen (guided by ultrasound). The sample is then sent for testing. Amniocentesis can be done from 15 weeks of pregnancy onward. Chorionic villus sampling (CVS) takes a sample of the placenta using a needle through the abdomen (again, guided by ultrasound). The sample is sent for testing. CVS can be done from 11 weeks of pregnancy onward. Serum screening can be done on a blood test between 15 and 20 weeks of pregnancy, that measures the quantity of pregnancy hormones in the mother’s blood. Nuchal translucency, done between 11 and 14 weeks, uses ultrasound to measure the fluid space at the back of the fetus’s neck. Combining serum screening with the nuchal translucency test along with the age of the mother can give a fairly accurate calculation of the risks of having a child with a chromosomal abnormality. With in vitro fertilization (IVF), blastocysts can undergo preimplantation genetic screening (PGS) to test for chromosomal abnormalities prior to transfer to a uterus.

While there are no cures for chromosomal abnormalities, some treatments do exist to help treat the conditions that can result.

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