etting Pregnant and Fertility Tests: Pre-Implantation Genetic Diagnosis (PGD)
In the process of in vitro fertilisation (IVF), embryos are created in situ and then transferred to the uterus. While this assisted reproduction process helps to increase a couple’s chances of getting pregnant, it also raises concerns with regard to the chromosomal makeup of the embryos used.
One method that has been developed in order to assess chromosomal abnormalities is a type of testing known as pre-implantation genetic diagnosis (PGD). But what exactly is PGD? And what are the advantages of pre-implantation genetic diagnosis for couples using IVF in order to improve their chances of getting pregnant?
What is PGD?
Pre-implantation genetic diagnosis is a sophisticated diagnostic tool used to screen for a variety of genetic and chromosomal abnormalities.
There are two basic types of PGD tests. The first type screens for individuals with a known risk of genetic disease, including cystic fibrosis, Tay Sachs disease, muscular dystrophy and chromosomal translocations. The second type of PGD test analyzes for aneuploidy to check for common chromosomal irregularities related to advanced maternal age, repeated pregnancy loss and multiple IVF procedures.
It is important to be aware that PGD is offered in limited fertility centres; in fact, some fertility centres only offer this in vitro fertilisation test for couples who have experienced multiple pregnancies due to genetic reasons or couples who have had a pregnancy or child with genetic disease.
In addition to being available on a limited basis, pre-implantation genetic diagnosis is time sensitive and requires advanced preparation, meaning that couples should speak to their fertility specialist about beginning this process well in advance of starting IVF treatment.
What Does PGD Help to Detect?
Pre-implantation genetic diagnosis helps to test for the following genetic abnormalities:
|Charcot-Marie-Tooth disease||Cystic fibrosis||Down's syndrome||Duchenne muscular dystrophy||Franconi's anemia||Huntington disease||Marfan's syndrome||Muscular dystrophy||Sickle cell anemia||Spinal muscular atrophy type I||Tay Sachs disease||Thalassemia|
In addition, PGD also tests for chromosomal abnormalities, such as Down’s syndrome. It can also check for gender-specific abnormalities, such as male-specific diseases like muscular trophy, X-linked mental retardation and male-specific hemophilia. Couples should contact their local IVF centre to inquire as to whether PGD testing is available for a specific condition.
In PGD testing, a cell is generally removed from an embryo three days following fertilisation, when the embryo has grown to 8 cells. The cell is then examined for its genetic makeup using one of the following techniques:
- fluorescent in situ hybridisation (FISH): this process assesses the number of selected chromosomes
- polymenase chain reaction (PCR): this based DNA amplification process analyzes for genetic mutations
Test results are usually available within 24 hours and enable a couple and their fertility specialist to decide which embryos to transfer, which increases the chance of a healthy pregnancy.
PGD Success Rates
Because PGD is a fairly new procedure, long-term studies are not yet available with regard to PGD success rates.
However, the American Society for Reproductive Medicine and the Society for Assisted Reproductive Technology reports that a minimum of 1,000 babies have been born worldwide following PGD. The report also did not find any increased rates of malformation in fetuses that have been analyzed using PGD.
While the risk of having a pregnancy with a chromosomal irregularity is low (less than 2%), not all embryos produced will have the required chromosomal composition in order to result in a successful pregnancy and birth. In fact, most embryos will not develop or implant. Studies have demonstrated that a woman 30 years of age will have approximately one-third of her embryos which lack the required chromosomal composition.
As such, the natural pregnancy rate in fertile women is approximately 20 to 30% per month. As a woman gets older, chromosomal irregularities increase to that by the time she is 40 years, roughly 70% of a woman’s embryos will lack the appropriate number of chromosomes to result in a successful pregnancy. However, the risk of giving birth to a child with chromosomal abnormality is still very low, meaning that many of these embryos will stop developing at an early stage or miscarry if they are unable to implant. PGD for aneuploidy screening can help analyze for chromosomal abnormalities.
The cost of PGD is on average between £1002.40 and £2004.80.