Genetic Disorders and Female Infertility
Many women are unable to conceive and deliver a healthy baby due to genetic factors. Sometimes this is due to an inherited chromosome abnormality. Other times it is because of a single-gene defect passed from parent to child. Genetic disorders and diseases are introduced when the normal DNA sequence is altered in part or in whole, caused by mutation in one or multiple genes.
In addition, if other women in your family have had problems conceiving due to premature menopause, endometriosis or other factors, you may be at increased risk of the same problems.
Embryos with chromosome abnormalities have poor implantation rates. When they do implant, a miscarriage often occurs or the baby is born with physical issues, developmental delay or impaired intellectual function.
The kinds of chromosome abnormalities are:
- A missing piece of a chromosome (deletion).
- An upside-down chromosome (inversion).
- A change in the gene’s DNA sequence (mutation).
- Too many or too few chromosomes (aneuploidy).
- Chromosome pieces attached to the wrong chromosome (translocation).
Translocation is the most common of these. Although a parent who carries a translocation is frequently normal, his or her embryo may receive too much or too little genetic material, and a miscarriage often results.
Down syndrome is usually associated with advanced maternal age and is a common example of aneuploidy. Down syndrome is caused by having an extra number of the 21 chromosome (three instead of two). It is also referred to as trisomy 21.
Inherited genetic disorders
The existence of an inherited genetic disorder due to abnormal genes or mutations can also occur, though this is rare. Chromosome analysis of the parents’ blood identifies such an inherited genetic cause in less than 5% of couples.
Single-gene abnormalities are mutations caused by changes in the DNA sequence of a gene, which produce proteins that allow cells to work properly. Gene mutations alter the functioning of cells due to a lack of a protein.
Single-gene disorders usually indicate a family history of a specific genetic disease such as cystic fibrosis (CF) – an incurable and fatal disease affecting the mucous glands of vital organs – and Tay-Sachs disease, also a fatal disorder, in which harmful quantities of a fatty substance build up in tissues and nerve cells in the brain.
Though generally rare, these diseases are usually devastating to a family. Fortunately, much progress has been made in detection through PGD in conjunction with IVF.
Although a couple may otherwise have no fertility problems, IVF and PGD can work together to spare mother and father from heartache in cases where there is a known single-gene disorder in family history.
Embryo testing for genetic disorders
During IVF, a woman’s egg is fertilized and the embryos grow for several days. On the third day, healthy embryos are usually at the six or eight cell stage, so one cell is removed from each viable embryo to be biopsied and evaluated for genetic diseases and disorders through preimplantation genetic testing that includes PGD and PGS (preimplantation genetic screening). The embryo can compensate for the loss of one cell during biopsy, and it has been proven to not prevent embryonic development. One or more of the embryos that are established to be disease free are then transferred to the woman’s uterus.
Considering there is a known error rate for PGD that is performed before pregnancy, it is recommended that common methods of prenatal testing after pregnancy, like chorionic villus sampling (CVS) and amniocentesis, be performed in addition to PGD to eliminate any risk of misdiagnosis.
Aneuploidy screening checks for a specific set of chromosomes that contribute to recurrent miscarriages to make sure they are normal, which can be done during PGD and PGS. The most common chromosomal abnormalities related to pregnancy loss include:
- Trisomy (three copies of a chromosome).
- Monosomy (one copy of a chromosome) for chromosomes 13, 15, 16, 18, 21 or 22.
- Triploidy (three copies of all the chromosomes).
- Sex chromosomes abnormalities.
As with embryo testing, aneuploidy screening requires ovarian stimulation, egg harvesting, lab fertilization, embryo culture, and embryonic cell sampling. The cell removed from each embryo is tested for number of chromosomes or for a specific chromosome disorder. Only embryos that test normal are transferred to the uterus, expanding chances of a healthy pregnancy.