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Chromosomal Abnormality

Definition - What does Chromosomal Abnormality mean?

A chromosomal abnormality is when an embryo, fetus or person is missing a chromosome, has an extra chromosome, or has an abnormal section of a chromosome.

DNA, the instructions that program the development and functioning of bodies, are held within the structure of the chromosomes. If there is a chromosomal abnormality, developmental instructions may be missing or altered, which can lead to recurrent miscarriages, failed IVF attempts, unexplained infertility, and many genetic disorders.

FertilitySmarts explains Chromosomal Abnormality

In a typical embryo, half of the chromosomes come from the egg and half come from the sperm, for a total of 46 chromosomes.

Chromosomal abnormalities have two categories:

  • Having the wrong number of chromosomes (aneuploidy)
  • Having an abnormal chromosome structure

Aneuploidy

Aneuploidy could be having additional 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, may 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.

Abnormal Chromosome Structure

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

Testing for 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, which 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 the process of in vitro fertilization (IVF), blastocysts can undergo preimplantation genetic testing for aneuploidy (PGT-A) to test for chromosomal abnormalities related to copy number prior to transfer to a uterus.


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