Couples in which one of the partners carries a balanced chromosome rearrangement may find that starting a family is more difficult than they imagined. In fact, until they experience miscarriages or problems becoming pregnant, many individuals may not even be aware that they carry a balanced chromosome rearrangement. This article will answer the following questions:
- What are balanced chromosome rearrangements and who may be at risk to be a carrier?
- What are the different types of balanced rearrangements?
- What is an unbalanced chromosome rearrangement and why does it matter?
- How do you find out if you are a balanced rearrangement carrier?
- If you are a balanced rearrangement carrier, what are your pregnancy options?
Balanced Chromosome Rearrangements
Normally, humans have 23 pairs of chromosomes for a total of 46 in each cell. When a person carries a balanced chromosome rearrangement, it means their chromosomes are organized in a different manner than expected. About one in every 500 people in the general population is a balanced chromosome rearrangement carrier. Being a balanced chromosome rearrangement carrier is not a bad thing; it simply means you may have trouble conceiving or having a healthy child.
An example of a common rearrangement is an 11;22 translocation. In this case, a piece of chromosome 11 has exchanged places with a piece of chromosome 22. The person who carries this translocation is, in all likelihood, healthy and normal. They do not show any symptoms because all of their chromosome material is present; it is simply not located where we expect it to be located.
Unfortunately, problems can arise when this person decides to start a pregnancy, as the embryo may not inherit the correct amount of chromosome material needed to develop properly. Miscarriage is a common occurrence for people with balanced chromosome rearrangements – in fact, on average, about half of the pregnancies of a person carrying a balanced chromosome rearrangement will be lost in miscarriage because of unbalanced amounts of chromosome material in the embryo or fetus that do not allow it to develop properly.
Types of Balanced Rearrangements
There are several types of balanced chromosome rearrangements:
- Reciprocal translocations
- Robertsonian translocations
Reciprocal translocations occur when a piece of one chromosome has exchanged places with a piece of another chromosome but no chromosome material has been lost. The 11;22 translocation described above is an example of a reciprocal translocation.
Robertsonian translocations occur when two chromosomes stick together; therefore, individuals with a Robertsonian translocation have 45 instead of 46 chromosomes but they still have the correct amount of chromosome material.
Inversions are when a piece of a chromosome is flipped upside-down.
Unbalanced Chromosome Rearrangements
Individuals who carry a balanced chromosome rearrangement are at risk of having pregnancies or children with either too little or too much chromosome material. This situation is called an unbalanced chromosome rearrangement. Having too little or too much chromosome material would either cause miscarriage or result in the birth of a child with birth defects and cognitive disabilities.
Testing For Balanced Chromosome Rearrangements
Couples who have had three or more miscarriages have about a 5% chance for one member to carry a balanced rearrangement. If you and your partner have experienced three or more miscarriages, you should talk to your doctor about checking your chromosomes through a simple blood test called a karyotype.
Options for Balanced Rearrangement Carriers
Many couples in which one of the partners is a rearrangement carrier choose to conceive naturally and, if the pregnancy continues past the first trimester, they may choose to undergo prenatal testing in the form of chorionic villus sampling (CVS) or amniocentesis to confirm that the baby has normal chromosomes. Some couples may select to use either a sperm or egg donor to avoid the risk of a pregnancy with an unbalanced chromosome rearrangement, while other couples may elect to adopt a child. Other couples decide to pursue In Vitro Fertilization (IVF) with Preimplantation Genetic Diagnosis (PGD) to select embryos that have the correct amount of chromosome material.
IVF with PGD
In Vitro Fertilization (IVF) is a laboratory process in which a woman’s eggs are stimulated to develop, then removed and combined with the man’s sperm outside of the body. Preimplantation Genetic Diagnosis (PGD) is testing performed on a single cell of an early embryo to determine its chromosome make-up before that embryo is transferred back into a woman’s uterus. This testing allows the couple and the IVF doctor to choose to transfer onlyy those embryos with the normal amount of chromosome material. PGD is always performed in conjunction with IVF.
When a balanced chromosome rearrangement carrier chooses to have IVF with PGD, their doctor and the PGD laboratory will review their specific chromosome rearrangement. Egg retrieval and fertilization will occur under the guidance of the IVF doctor. A few days after fertilization, an embryo biopsy will be performed. The biopsied cells are sent to the PGD laboratory for testing. Only a single cell has to be removed to tell if an embryo has the correct amount of chromosome material (balanced chromosomes) or if it has either too much or too little chromosome material (unbalanced chromosomes). By only transferring those embryos that have balanced chromosomes, a couple can increase their chance of having a successful pregnancy and decrease the chances for miscarriage or the birth of a child with a chromosome abnormality.