Becoming a parent can be an exciting journey filled with excitement and anticipation. But for some, the path may take unexpected turns, including challenges with fertility. Aneuploidy, a condition that affects embryos and can make it difficult to conceive or carry a pregnancy, is one of them.
This article covers what aneuploidy is, how it happens, the different forms it can take, and how it’s detected.
What is aneuploidy?
Aneuploidy is a genetic condition where an embryo has an abnormal number of chromosomes in its genome, the “building blocks” of our traits and development.
Human cells are typically diploid, meaning they have 46 chromosomes arranged in 23 pairs. However, an embryo with aneuploidy may have one extra chromosome (trisomy) or one missing chromosome (monosomy).
When an embryo is missing or has an extra chromosome, it can lead to imbalances in gene expression, which can cause difficulty in conceiving, genetic disorders, and, in some cases, miscarriage.
Common genetic disorders caused by aneuploidy include:
- Down syndrome (trisomy 21)
- Turner syndrome (monosomy X)
- Klinefelter syndrome (XXY)
What is the normal embryo success rate?
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Types of Aneuploidy
The two main types of chromosome aneuploidies are trisomy (an extra copy of a chromosome) and monosomy (a missing copy of a chromosome).
Trisomy
An embryo with 47 chromosomes instead of the usual 46 is a trisomy. Trisomy 21, or Down syndrome, is the most well-known form of trisomy.
Other autosomal trisomies include:
- Patau syndrome (trisomy 13)
- Edwards syndrome (trisomy 18)
Monosomy
When an embryo has 45 chromosomes, it’s called monosomy. This is rare, and often means the embryo can’t survive.
One exception is Turner syndrome, which occurs when a female embryo is missing all or part of one of its X chromosomes. The X chromosome is one of the two sex chromosomes (X and Y) that determine a person’s biological sex. Females typically have two X chromosomes (XX), while males have one X and one Y chromosome (XY).
The risk of having aneuploid embryos increases with maternal age.
Other ploidy levels
In addition to trisomy and monosomy, there are rarer cases where an embryo might have an entire extra set of chromosomes (called polyploidy) or only a single set of chromosomes (called haploidy).
These cases are:
- Triploid: 69 chromosomes (an extra set)
- Tetraploid: 92 chromosomes (two extra sets)
- Haploid: 23 chromosomes (only one set)
A Closer Look at Aneuploid Embryos
Aneuploidy can happen at any point in life, but it usually occurs during cell division, either during meiosis, when gametes (eggs and sperm) are formed, or in mitosis as the embryo develops. Sometimes, an embryo can end up with an extra or missing chromosome.
While the prevalence of aneuploidy is relatively high, affecting roughly 10% of pregnancies, it can cause serious complications for the expectant mother and developing fetus, including an increased risk of miscarriage. About half of all pregnancies with a chromosomal abnormality result in miscarriage.
It’s important to remember that aneuploidy is not a condition that can be corrected or “fixed” during pregnancy, and pregnancy loss is never your fault. About 15-25% of known pregnancies end in miscarriage, and in the first trimester the majority of losses is due to aneuploidy. This is because having an abnormal number of chromosomes can make it difficult for an embryo to develop.
Aneuploidy and IVF
Embryos created through in vitro fertilization (IVF) can be at risk for aneuploidy as well. This is partly due to the nature of IVF itself, where fertilization and early embryo development happen outside the body.
While IVF has helped many couples conceive, it can also increase the risk of aneuploidy.
Here are a few examples of how IVF may contribute to aneuploidy:
- Intracytoplasmic sperm injection (ICSI): ICSI involves injecting a single sperm directly into an egg. Although ICSI can be a helpful fertility treatment option for aspiring parents with male factor infertility or those who have had unsuccessful IVF cycles in the past, some studies suggest that ICSI might increase the risk of aneuploidy compared to conventional IVF. This is because ICSI bypasses some of the natural selection processes that occur during fertilization.
- Embryo culture conditions: The environment that an embryo is cultured in can also influence aneuploidy. Factors such as temperature, pH, and nutrient levels can all affect embryo development and potentially increase the risk of chromosomal abnormalities.
Evaluating Embryo Quality and Ploidy
During IVF, embryologists carefully examine embryos under a microscope to assess their quality and characterization. They look at factors like the number of cells, cell symmetry, and the degree of fragmentation.
Embryos are often graded based on appearance, with those that appear the most “ideal” receiving higher grades. However, it’s important to note that there can be variability in how embryos are graded, as different embryologists might have slightly different criteria and scoring systems.
Are high-quality embryos always euploid?
A “good-looking” embryo doesn’t necessarily mean it’s chromosomally normal. While euploid embryos are more likely to develop into high-quality embryos, an embryo can appear normal and still have aneuploidy. This could be due to DNA damage, chromosome errors, or other genetic issues that may affect its karyotype.
That’s why preimplantation genetic testing for aneuploidy (PGT-A) is so important. PGT-A techniques aim to accurately determine an embryo’s chromosomal status, regardless of its visual appearance. This helps to identify embryos with the best chance of implantation and a healthy pregnancy.
Understanding Aneuploidy and PGT Results
If you’re undergoing IVF or another assisted reproductive technology (ART), your doctor might recommend preimplantation genetic testing to help screen embryos for chromosomal abnormalities, including aneuploidy.
While PGT-A can provide helpful information, it’s not a foolproof test. While it can accurately detect many chromosomal abnormalities, there’s still a chance of misdiagnosis or other genetic issues that might not be detected.
Other types of PGT, including PGT-M (for monogenic disorders) and PGT-SR (for structural rearrangements), can also screen for specific genetic conditions.
Here’s a quick overview of the terms you might hear when discussing your PGT results:
- Aneuploid: An embryo with an abnormal number of chromosomes.
- Euploid: An embryo with 46 chromosomes.
- Mosaic: An embryo with a mix of both normal and abnormal cells.
What causes aneuploidy?
Aneuploidy occurs when there’s an error during cell division and chromosomes fail to separate correctly into daughter cells. (known as chromosome missegregation).
This can happen during:
- Meiosis: when sex cells (sperm and eggs) are created. During meiosis, each chromosome makes a copy of itself (called a chromatid). Normally, these chromatids are supposed to separate evenly into the daughter cells. But sometimes, errors occur, and the daughter cells end up with an extra or missing chromatid. This results in sex cells with an abnormal number of chromosomes.
- Mitosis: when cells divide to grow and repair tissues in the body. Errors during mitosis in the early embryo can lead to mosaicism.
The most common causes of aneuploidy include:
- Advanced maternal age
- Meiosis I errors
- Genetic recombination
Exposure to certain environmental factors can also contribute to aneuploidy.
These include:
- Smoking
- Radiation or chemical exposure
- Viral infections
The Future of Aneuploidy Research and Treatment
Aneuploidy can also occur in other cells (somatic cells) and is associated with certain diseases and disorders. For example, aneuploidy is a common characteristic of cancer cells. The abnormal number of chromosomes in these cells can disrupt normal cell growth and behavior, contributing to the development and progression of cancer (tumorigenesis).
This is why being able to detect aneuploidy is so important — not just for fertility and pregnancy but for understanding the role it plays in other diseases.
Ongoing research is exploring new ways to prevent and treat aneuploidy through methods like gene editing, improved IVF techniques, and better preimplantation genetic testing. This research may also lead to the development of inhibitors that can target the specific pathways involved in chromosome missegregation, potentially reducing the risk of aneuploidy.
As of right now, prenatal tests such as amniocentesis and chorionic villus sampling (CVS) can identify chromosomal abnormalities in the developing fetus, but these invasive procedures carry risks such as miscarriage.
Takeaway
Aneuploidy is a common condition where an embryo has one extra chromosome (trisomy) or is missing a single chromosome (monosomy). It can cause difficulties in conceiving, genetic disorders, and miscarriage.
Thorough preimplantation genetic testing can help identify chromosomally healthy embryos, increasing the chances of having a baby.
While aneuploidy can cause problems with pregnancy, it’s also linked to other health conditions like cancer. Scientists are working hard to find new ways to detect and prevent aneuploidy.
A Word from RMA
If you’re worried about aneuploidy or have had past experiences with infertility or pregnancy loss, please don’t hesitate to reach out to our team of fertility specialists.
We’re here to help new and existing clients understand their options and develop a personalized treatment plan to give them the best chance of having a healthy baby.
Request an appointment or give us a call today. The first consultation is always free.
