What is Preimplantation Genetic Screening (PGS)?
Pre-implantation Genetic Screening (PGS) is a method of screening (testing) embryos for “chromosomal abnormalities” prior to being placed in the womb following IVF.
. Implanting normal embryos after PGS results in pregnancy rates >60%.
. PGS reduces the risk of miscarriage.
. As only normal embryos are selected, they can be frozen for your future use.
Read more about : PGD
About Iranian Surgery
Iranian surgery is an online medical tourism platform where you can find the best gynaecologists and obstetricians in Iran. The price of an Infertility treatment in Iran can vary according to each individual’s case and will be determined based on in-person assessment with the doctor.
For more information about the cost of Pre-implantation Genetic Screening (PGS) in Iran and to schedule an appointment in advance, you can contact Iranian Surgery consultants via WhatsApp number 0098 901 929 0946. This service is completely free.
Read more about : What are the Biggest Differences between IUI and IVF?
Before Preimplantation Genetic Screening (PGS)
What are chromosomes?
Chromosomes are the packages that carry our genetic information we normally have 46 chromosomes (23 pairs). The genetic information carried by the chromosomes is our body’s instruction manual for how we grow, develop and function.
What happens if an embryo has an incorrect number of chromosomes?
It is expected that a proportion of all embryos formed will be chromosomally abnormal. They are known as aneuploid embryos. This is true whether the embryo was formed through natural conception or through IVF treatment. The likelihood of an embryo carrying a chromosomal abnormality (aneuploidy) increases with age. Typically in women who are 35-37 50% of embryos will be chromosomally abnormal.
Some embryos may carry an extra copy of a chromosome, whereas some embryos may be missing a chromosome, in some cases there is complete duplication of a full set of chromosomes. A chromosomally abnormal embryo is less likely to implant and more likely to result in a miscarriage. Chromosomally abnormal embryos rarely result in a livebirth and if they do can be associated health problems for the child (e.g. Down’s syndrome).
In some cases, an embryo with the wrong number of chromosomes will spontaneously stop growing during its development in the laboratory and will not form a blastocyst (day 5/6) stage embryo. These embryos would not normally be considered for transfer. However, the majority of abnormal embryos will not stop growing until they have been transferred into the uterus. It is difficult for the laboratory to determine whether embryos are normal or abnormal from appearance alone. As such, it is often the case than an abnormal embryo would be transferred to the womb instead of a normal embryo, which may explain some unsuccessful IVF treatments.
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Specifically PGS can identify chromosomal abnormalities such as:
. Incorrect number of chromosome or aneuploidy – Aneuploidy is the presence of an abnormal number of chromosomes in a cell, such as having 45 or 47 chromosomes when 46 is expected in human cell. Examples of aneuploidy are:
. Down Syndrome with an extra copy of chromosome 21,
. Edwards Syndrome with an extra copy of chromosome 18.
. Translocation or rearrangement of one segment of chromosome onto another chromosome.
. Deletion or missing a segment of chromosome, or
. Sex-chromosome abnormalities including duplication or deletion of X and Y chromosome. Examples of sex-chromosome abnormalities include:
. Klinefelter Syndrome with three copies sex chromosome, two X and one Y
. Turner Syndrome with only one copy of sex chromosome, one X
Who Should Consider PGS?
Because PGS screens embryos for chromosomal abnormalities, it identifies an embryo with normal chromosomal makeup with much greater precision than visual microscopic observation typically used in IVF. Hence PGS allows the selection of a normal embryo to be transferred and increase the likelihood of delivering a healthy baby.
Couples or individuals at increased risk for having embryos with abnormal chromosomes are best candidates for PGS. These include:
. Advanced maternal age (age 35 and older),
. Recurrent miscarriages, or
. Previous failed IVF attempt(s)
Other appropriate candidates for PGS are individuals or couples who desire
. Single embryo transfer to avoid multiple gestation,
. A desire to maximize the possibility of a healthy baby with IVF, or
. Family balancing
Read more about : 2nd iui success rate
What are the risks of having PGS?
. No embryos for biopsy. There is a chance that no embryos develop on to the blastocyst stage and therefore that there are no embryos for biopsy and transfer. However, it is very likely that embryos that fail to develop to the blastocyst stage would be chromosomally abnormal.
. Embryo damage. There is a risk of embryos being damaged during the biopsy process meaning they are not suitable for freezing and transfer. This risk is very small (typically less than 1%).
. No normal embryos. There is a chance that all the embryos biopsied are aneuploid and therefore that there is no embryo suitable for transfer. This become more likely as female age increases.
. Risk of misdiagnosis. Unfortunately tests are rarely 100% accurate and there is a risk of a euploid embryo being incorrectly diagnosed as aneuploid and an aneuploid embryo being diagnosed as being euploid. The chances of this are less than 1%. We would always recommend prenatal screening for chromosomal abnormalities after PGS.
. Risk of no diagnosis/partial diagnosis. Some embryos may have no diagnosis, due to the absence of chromosomes, or technical difficulties in the fixation process. Embryos without a result can still be transferred, but the possible advantages of PGS will not apply. In addition, sometimes the analysis may not be clear for one of the chromosomes tested. Embryos with such partial results may be transferred, but this must be discussed with either a geneticist or a consultant. The risks of having such an embryo back will be explained to you.
. Unfortunately PGS does not guarantee a pregnancy or a healthy live birth nor does it eliminate the risk of miscarriage.
Worldwide several thousand babies have now been born from IVF with PGS, with no reported increase of congenital abnormalities above the general population rate of 3-5%.
During Preimplantation Genetic Screening (PGS)
How Does PGS Work?
There are several testing methods to screen all 24 chromosomes, also called comprehensive chromosomal screening (CCS). Currently the most common methods for CCS are:
. Array-Comparative Genomic Hybridization or aCGH, and
. Next-Generation Sequencing or NGS with higher resolution by probing more data points may soon replace aCGH.
Both methods are performed on several cells from a day 5 or day 6 embryo, or less commonly on one or two cells from a day 3 embryo.
What are the Steps Involved in PGS?
The entire process of PGS consists of multiple steps, each step performed by different experts and laboratories.
. The first part is in-vitro fertilization (IVF) with either conventional IVF or intracytoplasmic sperm injection (ICSI) by which embryos are produced. Fertilized embryos are cultured for 3 to 5 days.
. The second part is embryo biopsy. Embryo biopsy is performed on the third or fifth day of embryo development
. In Day 3 embryo biopsy, one to two cells are removed for testing, or
. In Day 5 embryo biopsy, several (3 to 6) cells are removed for testing.
Cells within a day 5 or day 6 embryo have separated into two types: cells which will form the fetus (inner cell mass) and cells which will form the placenta (trophectoderm). More cells can be removed at this stage (from the trophectoderm) without compromising the viability of the embryo, possibly leading to a more accurate test.
Biopsy of a day 5 embryo
. After the cells are biopsied and placed in test tubes, the tubes are transported by same-day overnight courier to laboratory for analysis.
. Usually it takes over 24 hours before the results are available. Hence freezing of the embryos using a rapid freeze method called vitrification is necessary.
. The final step, the thawing of a frozen embryo, typically a single embryo with normal PGS result, in subsequent cycle after the in-vitro fertilization procedure, and the transfer of the embryo back into the uterus is performed.
After Preimplantation Genetic Screening (PGS)
What is the chance of having a baby with PGS?
The chance of a successful outcome with PGS in conjunction with IVF treatment depends on whether any and how many of the embryos produced in an IVF cycle have a normal PGS result. Current data report a live birth rate between high 60% and 80% when an elective single frozen embryo is transferred in a subsequent cycle. These data are very encouraging as most women undergo PGS and IVF are at higher risk of producing chromosomally abnormal embryos such as advanced maternal age, previous IVF failures or recurrent pregnancy losses.
Sometimes, however, no embryos are suitable for transfer to the womb, for reasons including:
. Not enough eggs are produced or fertilized,
. Removing the cells to be analyzed damages the embryos, or
. All the embryos are aneuploid.
Is Prenatal Testing necessary after PGS?
Because PGS is carried out on a minute amount of tissue, the test can never be 100% accurate. As there is a small chance of an incorrect diagnosis, it is strongly recommended that prenatal testing (for example amniocentesis or chorionic villus sampling – CVS) is used to confirm that the fetus has normal chromosomal makeup. Amniocentesis and CVS collect many thousands of cells from the fetus and are therefore more accurate.
Does PGS test for a specific genetic disease?
PGS test for chromosomal abnormalities only. Preimplantation genetic diagnosis or PGD tests for single gene mutations associated with genetically inherited disease, e.g. cystic fibrosis, sickle cell anemia, muscular dystrophy, Huntington’s disease, Fragile X syndrome and many more.