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What Are Chromosomes and Why Are They Important in IVF?

My Post (3)-1Most of us remember very little from our Biology 101 class we had to pass in order to finish high school or college. The terms were technical and the processes complex. However, now that reproduction has taken center stage in your life, you keep hearing some of these terms long since forgotten.

In this post, I'll clarify some of the mystery around one part of IVF, pre-implantation genetic screening (PGS). This testing looks at genes and chromosomes to determine an embryo's risk of miscarriage.

What IS a Chromosome, Anyway?

Chromosome

Chromosomes are thread-like structures located inside the nucleus of the cell. Each chromosome is made of protein and a single molecule of deoxyribonucleic acid (DNA). Passed from parents to their children, DNA contains the specific instructions that make each person unique. Changes in the number or structure of chromosomes in new cells may lead to serious problems, such as Down’s syndrome, repeated miscarriages, or an inability for an embryo to implant.

Each healthy person normally has 23 pairs of chromosomes (a total of 46). There are 23 types of chromosomes: 22 pairs of autosomes numbered from one to 22, and one pair of sex chromosomes (XX or XY). A female has a pair of X chromosomes (XX), and a male has an X and a Y chromosome (XY). Whether a person has XX or XY chromosomes is determined when a sperm fertilizes an egg.

Unlike the body's other cells, the cells in the egg and sperm — called gametes or sex cells — possess only one chromosome. All of the female’s eggs are X chromosomes, and the sperm has either X or Y chromosomes. The sperm are the variable factor in determining the sex of the baby. If the sperm carries an X chromosome, it will combine with the egg’s X chromosome to form a female embryo. If the sperm carries a Y chromosome it will combine with the females X chromosome to form a male embryo.

Now you are beginning to remember why you slept through biology in the first place, aren’t you? So, how does all of this relate to IVF and preimplantation genetic screening (PGS)?

Chromosomal Abnormalities - More Likely with Age

Sometimes an egg or sperm may have an abnormal number of chromosomes, which results in an embryo with aneuploidy. Anueploidy is used to describe an abnormal number of chromosomes present in the cell. The majority of embryos with aneuploidy will not implant in the uterus or will result in a miscarriage. The majority of first trimester miscarriages are the result of aneuploidy in the embryo.

There are cases, however, in which a baby can be born with an abnormal number of chromosomes. One of the most common examples of this is Down Syndrome, which is caused by having three copies of chromosome 21 (Trisomy 21). An extra copy of a chromosome is called a ‘trisomy.’ A missing copy of a chromosome is called ‘monosomy.’

Other chromosome abnormalities may be translocations, where a chromosome or piece of chromosome has an incorrect position, or extra or missing segments of a chromosome, called deletions and duplications, which can also cause miscarriage or birth defects and intellectual disabilities.

Aneuploidy can happen in any pregnancy, but the chances increase as a woman ages. Because many embryos with aneuploidy are miscarried, the chances of miscarriage also increase as a woman ages.

How We Conduct Pre-Implantation Genetic Screening (PGS)

In PGS, cells from the trophectoderm of the embryo (the cells that eventually make up the placenta) are biopsied from the embryo and sent for analysis. PGS is done prior to the embryo transfer, therefore, once the embryos have been tested, only embryos with the correct number of chromosomes will be transferred, increasing the likelihood of success.

PGS only tests for the NUMBER of chromosomes present in embryos. PGS does NOT test for single gene disorders (Cystic Fibrosis, Tay Sachs, etc,.). This is probing much deeper into the DNA of the chromosome (much like a needle in a haystack) and requires more information about the specific condition prior to testing the embryos.

For further information about PGS and see if this resource may help your chances of building a family, make an appoinment at one of InVia's four Chicago area fertility clinics.

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IVF InVia Fertility Specialists Embryology

Liza Roscetti Meyer

Liza Roscetti Meyer

Liza has been an embryologist at InVia Fertility Specialists since 2002. She completed her studies, first in animal science at Southern Illinois University, followed by clinical embryology/reproductive genetics at Eastern Virginia Medical School. Her interests include reproductive genetics, fertility preservation, and mitochondrial function in the human oocyte. She loves being a part of helping others achieve and experience the joy of becoming a parent. Seeing the end result .your baby.makes all the hours in the lab worth it!

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