Recurrent pregnancy loss (RPL) is defined as two or more consecutive clinical pregnancy loss before the pregnancy reaches 20 weeks of gestation. The incidence of RPL is around 1–2% in couples who are desirous to conceive. Various causes of RPL include advanced maternal age, uterine abnormalities, chromosomal abnormalities, genetic abnormalities in the baby, immune disorders, and/or hormonal imbalances. Recently, much attention is being paid to genetic causes involving changes in parental or fetal chromosomes. Of recurrent miscarriages, 3–6% are due to chromosomal abnormalities of one of the two partners.
What are genes and chromosomes?
Our bodies are made up of millions of cells. Usually, we have 46 chromosomes in most cells. We inherit our chromosomes from our parents, 23 from our mother and 23 from our father, so we have two sets of 23 chromosomes or 23pairs. Because the chromosomes are made up of genes we, therefore, inherit two copies of most genes, one copy from each parent. This is the reason why we often have similar characteristics to our parents. The chromosomes, and therefore the genes, are made up of a chemical substance called DNA. Genes act like a set of instructions, controlling our growth and how our bodies work. They are responsible for many of our characteristics, such as our eye color, blood type, or height. Genes are carried on thread-like structures called chromosomes.
It is important that we have the correct balance of chromosome material. This is because the genes, that instruct the cells in our body, are found on the chromosomes. Any change in the number, size, or structure of our chromosomes can mean a change in the amount or arrangement of genetic information. A change in the amount or arrangement of genetic information may result in learning disability, developmental delay, and health problems in a child.
There are two main types of chromosome changes that can occur.
1. Changes in chromosome number: This is when there are more or fewer copies of a particular chromosome than usual.
Usually, every cell in a person’s body contains 46 chromosomes. Sometimes however a baby is born with either too many or too few chromosomes. The baby, therefore, has too few or too many genes or instructions. One of the most common examples of a genetic condition caused because of an extra chromosome is Down syndrome. People with this condition have 47 chromosomes in their cells instead of 46. This is because there are three copies of chromosome number 21 instead of the usual two.
2. Changes in chromosome structure: This is when the material in an individual chromosome is disrupted or rearranged in some way.
Changes in chromosome structure happen when the material in an individual chromosome is broken and rearranged in some way. This may involve the addition or loss of chromosome material. This may happen in a number of ways which are discussed below.
Changes in chromosome structure may be very subtle and hard to detect by scientists in the laboratory. Even when the change in structure is found, it is often hard to predict what effect the change will have on an individual child. This can be frustrating for parents who are keen to have as much information about their child?s future as possible.
Types of mutations that cause changes in Chromosomal Structure:
Translocation is an unusual arrangement of the chromosomes in the cell. Most people with a translocation have the correct amount of genetic material but it is arranged in an unusual pattern. In the case of Robertsonian translocations, two whole chromosomes become ‘stuck together’ resulting in 45 chromosomes instead of the usual
The term chromosome deletion means that part of a chromosome has been lost or deleted. A deletion can happen on any chromosome, and along any part of the chromosome and can be any size. If the material (genes) that have been deleted contains important instructions for the body, that person may have a learning disability, developmental delay, and health problems. The seriousness of these depends on how much of the chromosome has been deleted, and where the deletion is.
The term chromosome duplication means that the chromosome has duplicated part of itself so that there is too much chromosome material present. This additional chromosome material may mean there are too many instructions for the body to process, and this may result in learning disability, developmental delay, and health problems in a child.
Chromosome inversion means that part of a chromosome has turned so that the sequence of genes in the chromosome is partly reversed. In the majority of cases, this does not cause any health problems to the person carrying the inversion.
The term chromosome ring means that the ends of a chromosome have joined together in a "ring? shape. This usually happens when the two ends of the same chromosome are deleted. The remaining ends of the chromosome are „sticky? and join together to make a ring shape.
Chromosome insertion means that material from a chromosome has been inserted into an unusual position in the same or another chromosome. If there is no additional or missing chromosome material, that person is usually healthy. However, if there is additional or missing chromosome material, then that person may have a learning disability, developmental delay, and health problems.
If a parent has an unusual chromosome rearrangement, what will happen to their child?
There are several possibilities for each pregnancy:
- The child may inherit an entirely normal chromosome arrangement.
- The child may inherit the same chromosome rearrangement as the parent.
- The child may be born with learning disabilities, developmental delays, and health problems.
- The pregnancy ends in miscarriage.
Therefore it is usually possible for a person who carries a chromosome rearrangement to have healthy children, and many do. As each rearrangement is unique, carriers would have to discuss their particular situation with a genetic specialist.
Points to remember
- A rearrangement is either inherited from a parent or happens around the time of conception [de novo]
- A rearrangement cannot be corrected – it is present for life.
- The vast majority of carriers of a balanced rearrangement are able to have healthy children.
Who does Genetic Counseling Help in the management of RPL?
At Care IVF, couples with a history of multiple miscarriages are generally referred to our inhouse - geneticist for detailed genetic counseling. (read genetic counseling FAQs )
- A genetic counselor's role is to check if genetic factors are indeed the cause and if yes the next step wd be to identify the type of mutation the couple may be carrying.
- This would in turn help in pre-implantation genetic screening to screen the embryos for the identified mutation and sort out normal embryos for implantation.