Preimplantation Genetic Testing
What is PGD Genetic Screening in IVF?
Preimplantation genetic diagnosis (PGD) and preimplantation genetic screening (PGS) are the two types of genetic tests that evaluate embryos created through in vitro fertilization (IVF) for genetic defects before they are transferred to the uterus.
Genetic defects in embryos can
- make it difficult to conceive,
- result in miscarriage, and
- may result in unhealthy children.
Preimplantation genetic testing can identify healthy IVF embryos with the greatest chance of implanting in the uterus, ensuring higher chances of achieving a healthy pregnancy and greatly reducing the chance of birth defects in a child.
What is Preimplantation Genetic Diagnosis (PGD)
PGD evaluates cells extracted from an embryo for specific genetic mutations in the DNA sequence that can result in diseases. PGD is used to identify whether the DNA in an IVF embryo has a specific genetic disorder that one or both parents may carry or be at risk for passing to their children.
Individuals and couples who are known carriers, or have a family history of genetic disorders, have an increased risk of passing that disorder to their children. People don’t necessarily have to have a genetic defect to pass it on to their child, so family histories can be especially important.
PGD assesses the embryo’s DNA to see if a parent’s genetic mutation has been passed to it. This can be a mutation that causes a disease or one that predisposes an offspring to developing that mutation. For example, the gene for cystic fibrosis can make pregnancy unlikely and could be passed on to the child, and the BRCA1 & 2 genetic mutations predispose a female child to developing breast and/or ovarian cancer.
DNA carries the specific genes an embryo has inherited and also the “instructions” for how the genes are assembled. These control the development and growth of the embryo and the resulting child.
The Purpose of Preimplantation Genetic Testing
According to the American Society for Reproductive Medicine, “PGD involves testing an embryo before it implants for a specific, known genetic disorder.” Preimplantation genetic testing can be used to ensure that the embryo selected for transfer has the correct number of chromosomes, thereby reducing the chances for a failed IVF cycle and reducing the chance of miscarriage.
To identify whether the embryo has a particular genetic mutation, doctors will develop a ‘probe’ (a DNA sequence) specific to that condition. Doctors typically have identified a relevant condition in advance based on the parents’ and/or their families’ medical histories and have usually confirmed it from them in advance. PGD can identify more than 100 different conditions, but it can only evaluate an embryo for one genetic disorder at a time, making it essential to know which genes may be abnormal prior to embryo biopsy.
The embryo is biopsied during its early development in the lab after egg retrieval and fertilization – at day 5 or 6 after egg retrieval. The cells gathered are analyzed in a lab.
PGD cannot completely eliminate the possibility that the child will inherit a genetic disorder, but does meaningfully reduce this risk. If implantation is successful and a pregnancy results, patients who are at risk for passing on a genetic mutation may opt for further testing, such as amniocentesis, to identify genetic abnormalities.
Why Have Preimplantation Genetic Testing?
Testing embryos and implanting only embryos without defect into the mother’s uterus can help prevent the passing of hereditary diseases and improve the chances of a successful pregnancy.
The disorders and diseases most frequently identified by PGD are:
- BRCA1 & BRCA2 genetic mutations
- Cystic fibrosis
- Duchenne muscular dystrophy
- Fragile X syndrome
- Hemophilia A
- Huntington’s disease
- Myotonic dystrophy
- Sickle cell anemia
- Spinal muscular atrophy
- Tay-Sachs disease.
PGD is ideal for couples that are concerned about hereditary diseases and are known carriers for a particular gene disorder.
Related Information: In Vitro Fertilization (IVF) Fertility Treatment
How Does PGS Testing Work?
The PGS process is somewhat less complex than PGD because rather than searching for minute, specific gene disorders, PGS tests the embryo for multiple larger chromosomal abnormalities.
PGS tests all 23 pairs of chromosomes for too few chromosomes or too many chromosomes in each pair. This imbalance is called aneuploidy. The most common type of aneuploidy is known as Trisomy 21, Down syndrome, which occurs when there is one too many chromosomes in pair 21. Chromosomal aneuploidy is linked to birth disorders and to miscarriages, including recurrent pregnancy loss (RPL).
Typically, human cells have 23 pairs of chromosomes – 46 chromosomes total. During fertilization, the embryo typically receives 23 chromosomes from each parent.
Understanding Genetics in Fertility is kind of like the old game of Telephone
Women are born with all the eggs they will ever have (at least with today’s technology) and tend to ovulate their best eggs first.
This is called the “production line theory” in which the earliest generation copies are the most accurate.
Think of a game of telephone, where if you are two people down the line, you get the original message, but if you are 20 people down the line, you have no idea what was originally said, because others have changed the message in passing it along.
Eggs that are later generation copies have more aneuploidy ( the occurrence of one or more extra or missing chromosomes ) , so there is a higher rate of birth defects, miscarriages and infertility as women age. Though sperm also accumulate defects with advanced paternal age, these tend to be single gene disorders (pinpoint) rather than defects in a whole or a large part of a chromosome.
Aneuploid embryos are the result of unequal division of the egg or sperm cells during fertilization; the resulting disorders do not typically run in families but rather occur spontaneously within developing eggs and embryos. Common forms of aneuploidy are monosomy, a single chromosome where there ought to be two, and trisomy, a third (extra) chromosome where there ought to be two.
Why Have PGS Testing During IVF?
As high as 80 percent (and some even argue 90 percent) of miscarriages are due to chromosomal abnormalities. PGS testing gives doctors the ability to identify genetically normal embryos that are more likely to result in pregnancy to implant into the uterus. If an aneuploid embryo implants and the pregnancy survives, the child will likely be born with a chromosomal abnormality, such as Down Syndrome.
Preimplantation genetic screening can decrease the possibility of miscarriage and increase the odds that an elective single embryo transfer (eSET) will result in singleton pregnancy (eliminating the chance of multiples). This maximizes the chances for a safe, successful pregnancy with live birth.
As a result, there is improved chances of pregnancy and a decrease in the time and costs involved with additional IVF cycles.
PGS is ideal for couples that
- have experienced multiple miscarriages,
- had unsuccessful IVF treatments,
- individuals with unexplained fertility,
- couples with a history of chromosomally abnormal children, and
- women above the age of 35.
What are the Risks with PGS and Other IVF Genetic Tests?
Health risks to children born after PGS and PGD are no higher than for children born through regular IVF. Though IVF babies seem to have higher risks for birth defects than babies conceived spontaneously, this is often seen in the setting of comparing apples and oranges, with fertile women compared to infertile women. When women with subfertility conceive with IVF, oral medication, insemination or spontaneously after infertility, the risk of birth defects seems relatively similar. The risk of damage to the embryos during cell removal for both tests is extremely low.
Preimplantation genetic testing offers couples experiencing infertility improved chances at becoming pregnant, ultimately saving them the emotional cost of pregnancy loss and financial costs of additional IVF treatments (because embryo transfer of a nonviable embryo adds cost and delays conception). However, some people take issue with PGS and PGD for ethical reasons, and there are many strong opinions surrounding the issue.
Positive Steps Fertility Approach to PGD and PGS Testing
At Positive Steps Fertility, we provide all appropriate fertility treatments to help our patients build their families, including preimplantation genetic diagnosis and screening as medically indicated. We can help patients navigate the various considerations – including ethical and financial – associated with IVF genetic testing to help them find the balance that is right for them.
PGD & PGS Options at Positive Steps Fertility
Positive Steps Fertility is focused on transparency and clear communication. We want you to understand all of your options as you take these next steps toward building your family. Schedule a consultation today to learn more about our IVF genetic testing and other fertility treatment options.