Understanding Chromosomes
To understand what an aneuploidy is, you’ll need to first learn about chromosomes. Chromosomes are the structures that contain our DNA. They are located in the nuclei of all of our cells. Normal human cells have 46 chromosomes comprised of 23 chromosome pairs. Half of our chromosomes come from our mothers and the other half from our fathers. The first 22 chromosome pairs, called autosomes, are the same in males in females. The 23rd pair is comprised of the sex chromosomes. In females, this pair is two X chromosomes. In males, this pair is an X and a Y chromosome.
What Is an Aneuploidy?
In aneuploidy, a cell has three copies of a particular chromosome (resulting in 47 total chromosomes) or one copy of a particular chromosome (resulting in 45 chromosomes). An extra copy of a chromosome is called a trisomy; a missing copy is called a monosomy. Any change in the number of chromosomes in the sperm or egg cell can affect the outcome of a pregnancy. Some aneuploidies can result in a live birth, but others are lethal in the first trimester and can never lead to a viable baby. Before the tenth week of pregnancy, aneuploidies are the most common cause of miscarriages. Also Known As: Aneuploidies, Trisomy, Monosomy, Chromosome Abnormality
Causes
Environmental influences may be able to cause chromosomal abnormalities, but scientists believe that most cases of aneuploidy result from random errors in cell division. Most of the time, even when chromosomal testing after a miscarriage shows that the baby was affected by aneuploidy, the aneuploidy does not recur in future pregnancies.
How Trisomies Affect Pregnancy
Trisomies (an extra copy of a chromosome) are more common than monosomies (a missing copy). Most trisomies result in miscarriage. In fact, they are responsible for around 35 percent of miscarriages. Less than one percent of people are born with trisomies. There are significant health effects of the chromosome abnormalities in these individuals. Only a few trisomies can result in live births. The most common is trisomy 21, known as Down syndrome. The other two autosomal (non-sex chromosome) trisomies that sometimes result in a baby are trisomy 13 and 18. Unfortunately, babies born with these trisomies usually do not survive. Babies born with certain sex chromosome trisomies can live to adulthood. Males born with two X chromosomes and one Y chromosome have Klinefelter’s syndrome, the most common aneuploidy after Down syndrome. Males can also survive with one X and two YY chromosomes and females with three X chromosomes. As for monosomies—the opposite of trisomies—only one results in live birth. This is a single X chromosome in females, known as Turner’s syndrome.
