We increase your chances of success in IVF with Next Generation Sequencing (NGS).
In this section, we tried to give answers to questions about NGS and PGS.
1. What is Preimplantation Genetic Screening (PGS)?
There are 46 chromosomes, 22 pairs of autosomal (1, 2, 3, 4,…..,22) and 2 gender-determining gonadal chromosomes (XX or XY) in a healthy person. There are 23 chromosomes, as being 22+X or 22+Y in each egg and sperm, which are reproductive cells, and individuals with 46 chromosomes are formed by the union of reproductive cells containing 23 chromosomes. The earliest phase of human development is embryonal development, and this period can be observed for nearly 5-6 days in IVF.
But do embryos always contain 46 chromosomes?
Unfortunately, the answer to this question is “no”. Embryos developing both normally and in vitro may contain numerical or structural chromosomal abnormalities, and these abnormalities can often cause early miscarriage or health problems in the developing baby, compatible with life or not.
Detection of numerical (aneuploidies) and structural abnormalities (translocations, deletions and inversions) that may develop in embryos is nowadays possible with preimplantation genetic screening (PGS) methods. Aneuploidy screening with PGS method, in other words, 46 chromosomes can be examined in embryos obtained by in vitro fertilization methods. The PGS method helps to detect the chromosomally normal embryo and to obtain high continuing pregnancy rates with the transfer of this embryo.
In conclusion, the PGS method allows embryos to be examined in terms of chromosomal aneuploidies and translocations before transfer.
In our center, New Generation Sequencing (NGS) method, which is the most advanced of the effective genetic screening tests for PGS, is used. With the NGS method, aneuploid screening of the embryo can be performed with at least 95% accuracy in a small amount (1-5) of cells taken from the embryos by the embryo biopsy method. Embryo biopsy can be performed on embryos with higher than a certain quality that can reach the blastocyst stage. Hence, laboratory conditions should be sufficient to provide blastocyst embryo culture conditions.
2. What are the benefits of PGS and to whom is it recommended to be performed?
Embryos obtained in IVF are evaluated according to criteria defined based on their morphological developmental characteristics, and a healthy pregnancy is tried to be achieved by means of transferring the embryo with the best developmental characteristics to the uterus. Examination of the developmental characteristics of the embryo under the microscope does not give us the information that the embryo is chromosomally normal or abnormal. This information can only be obtained by genetic screening with PGS. Even if the embryos are of very high-quality, they may contain some chromosomal problems such as aneuploidy, and these problems may cause early miscarriage, some structural defects in the developing fetus, or the inability of the embryo to attach to the uterus.
Even if the embryos are of very high-quality, they may have some chromosomal disorders such as aneuploidy, and these disorders may cause early miscarriage, some structural defects in the developing fetus, or the inability of the embryo to attach to the uterus.
The main purposes of the application of PGS in embryos are to increase the adhesion of the embryo to the uterus after transfer, which is detected to be chromosomally normal, to reduce the possibility of miscarriage that may develop after attachment, and ultimately to increase the probability of couples to have a healthy baby.
So, who should be offered PGS?
It is known that more than 50% of spontaneous abortions are of chromosomal origin occurring in embryos. This rate increases especially in older women and men with very poor sperm counts (severe male infertility).
In our center, NGS-based PGS is recommended especially for the mentioned indications. Because the incidence of chromosomal disorders is higher in embryos developing in couples with these indications.
- Especially women with advanced maternal age (over 35)
- Women with recurrent miscarriages of unknown cause
- Women who do not get pregnant despite transferring quality embryos in 2 or more IVF attempts
- Men with very severe numerical and morphological problems with sperm values
- Men or women with gonadal mosaicism
- Couples in which the cause of the miscarriage is a chromosomal disorder in the baby/fetus
- Couples with structural chromosomal disorders such as translocation and inversion
3.What are the advantages of Preimplantation Genetic Screening with NGS?
- Low rate (below 5%) of false positive or false negative results.
- Higher accuracy of NGS compared to studies performed with Array cGH (comparative genetic screening).
- Ability to detect mosaicism that can develop in embryos.
- Ability to detect the imbalances (translocations, inversions and deletions) in chromosomes within all chromosomes at the same time.
- Allows mitochondrial scoring (mitoscoring).
- It shortens the period of having a healthy baby since it prevents miscarriages that may occur during pregnancy due to chromosomal abnormalities.
Frequently Asked Questions
1.What kind of procedure is performed on the embryo for NGS and do these procedures harm the embryo?
NGS-based preimplantation genetics is preferentially applied to embryos that have reached the blastocyst stage. The embryo biopsy method for NGS necessitates the removal of only a few cells from the trophoblast cells of the embryo after reaching the blastocyst stage, which will further form the placenta by differentiation. Embryos are tried to be defined chromosomally by genetic analysis made in these cells.
The embryo reaches the blastocyst stage, at the earliest, the 5th day after the egg collection. In cases such as advanced maternal age and severe male infertility, the development rate of embryos may be slower, and therefore it may take 6 or even 7 days for embryos to reach the blastocyst stage. Slow development does not increase the risk of chromosomal aneuploidy in embryos, at this point the developmental quality of the embryo is more decisive.
Embryos are also suitable for biopsy even on the 3rd and 4th days. However, during this period, the embryos may be mosaic. In other words, some of the cells may be chromosomally normal and some may be abnormal. This type of embryos can correct themselves in the future with their self-correction abilities, and turn into embryos with a completely normal chromosomal structure. If embryo biopsy is performed in the early period, the probability of giving false negative results, such as accepting a normal embryo as abnormal, increases. Therefore, the most accurate approach is to perform the biopsy of embryos in the blastocyst stage.
Embryo biopsy procedure is performed under the microscope with special pipettes and by embryologists with enough training and experience in this field. The duration of the procedure is short and does not cause any harm to the embryo. The biopsied embryos are then individually frozen using the ultra-fast freezing method called vitrification. Partial loss of viability or complete disruption/loss of viability after freeze-thawing procedure may be experienced in less than 5% of all embryos.
Published studies have not shown an increased incidence of birth defects in IVF infants transferred after performing biopsy and freeze-thaw, compared with IVF infants transferred without embryo biopsy and resulting in birth.
2. What genetic defects can be detected with NGS-based PGS?
First of all, it should be mentioned that NGS-based PGS is a preimplantation genetic screening method and can be used in the detection of numerical and structural imbalances in chromosomes. This test also has a certain limit. NGS-based PGS for aneuploidy screening cannot detect particularly familial single-gene diseases, and cannot rule out uniparental disomy, chromosomal imbalances including deletions and duplications below 6Mb (mega base). Also, PGS can detect some chromosomal set increases, polyploidies, but cannot detect whether the embryo is polyploid or not in polyploids where the sex chromosomes are also in their normal multiples (triploidy 69, XXX and tetraploid 92, XXXX or 92, XXYY), it assumes normal.
3. According to age groups, what is the probability of normality and of pregnancy with the transfer of a normal embryo after NGS-PGS?
It is known that the probability of an embryo being chromosomally normal decreases with increasing age. For this reason, pregnancy rates have been decreasing dramatically due to the increasing age of women, especially in IVF attempts without PGS. In the analyzes performed with NGS-PGS in women under the age of 40, the probability of the embryos to be normal is 40-60%, while this rate drops dramatically to below 20% in the over 40 years old group.
While the pregnancy rates of below 20% can be obtained with the transfer of an embryo that has not been genetically screened with NGS in women over the age of forty, the pregnancy rates close to 40-50% can be reached with the transfer of an embryo diagnosed as chromosomally normal with NGS.
4. If pregnancy occurs in the in vitro fertilization attempt in which the embryo with PGS was transferred, is prenatal genetic testing still recommended?
Although the accuracy of the NGS-based PGS method, in which numerical and structural chromosomal analysis is performed, is very high, there is still the possibility for the test not to detect some disorders and false positives. For this reason, with the recommendation of the doctor performing your pregnancy follow-up, invasive or non-invasive prenatal tests can be applied to detect the accuracy of either the PGS test or other genetic abnormalities that may develop during pregnancy which cannot be detected by the PGS test.