Reproductive medicine is closely related to medical genetics. The study of human genes allows doctors to identify the causes of infertility, including those of unclear etiology, to assess the likelihood of pregnancy, and also to obtain a prognosis for the development of both the embryo and the baby after birth.
There are many reasons for the manifestation of genetic pathologies in the unborn child: this is heredity, endocrine diseases and infections transferred by the mother, mutations that have arisen in the germ cells of parents with age, as well as the negative influence of the environment.
The doctor will assess the risks of a genetic or chromosomal pathology in the unborn child, study the history and information about your partner and your relatives, and determine the appropriateness of prescribing a genetic diagnosis.
Types of genetic research
At Scanfert Clinic you have the opportunity, if indicated, to undergo such studies as molecular genetic testing, preimplantation diagnostics, karyotyping, prenatal diagnostics. These procedures are not included in the cost of the IVF program and are paid for additionally.
Preimplantation genetic diagnostics (PGD)
Morphological assessment of embryos does not allow assessing their genetic status. In other words, the appearance of the embryo is not strictly correlated with its genetic material. Over 50% of morphologically good embryos have hereditary material abnormalities. This may explain some of the implantation failures when transferring morphologically good embryos.
For example, 90% of aneuploid embryos (with an abnormal number of chromosomes) stop developing early and are unable to implant. In the remaining 10% of cases, a biochemical pregnancy or miscarriage occurs. Very rarely, the result is the birth of a child with Down syndrome.
In order to exclude genetic abnormalities in embryos until the moment of their implantation, PGD is performed. Such a study allows you to select embryos with a normal number of chromosomes. PGD significantly increases the chances of a successful IVF outcome.
Absolute indications are the presence of sex-linked hereditary diseases, as well as a chromosomal abnormality in one of the spouses or relatives. In addition, among the indications:
- repeated IVF attempts that ended in failure;
- woman's age over 35 years;
- frequent contacts of a woman with toxic substances, previous radiation exposure or treatment with drugs that can cause mutations;
- man’s age over 49 years old;
- repeated spontaneous abortion, missed pregnancy history
- male infertility due to spermatogenesis problems.
Genetic analysis can be performed in a variety of ways. The aCGH and NGS methods provide the most reliable and informative results.
As a result, geneticists receive a description of the structure of nucleotides in RNA and DNA molecules, which is compared with standard data to identify deviations. This procedure is highly informative and accurate (99.99%).
For the study, a biopsy (collection of genetic material) of the embryo is carried out on the fifth or sixth day. In this case, several cells of the trophectoderm are used, from which extraembryonic tissues - the placenta - are subsequently formed, so that the manipulation is completely safe: the probability of negative consequences is less than 1%.
Such a study allows you to select for transfer embryos with a normal number of chromosomes.
One of the ways to find out the causes of infertility in a couple and the risk of chromosomal diseases in a child is karyotyping. This type of research includes the analysis of a set of chromosomes to identify their abnormalities.
The human genome has 46 chromosomes (or 23 pairs of chromosomes): 22 pairs of autosomal and one pair of sex chromosomes (two X chromosomes in women, X and Y chromosomes in men). Each of the two chromosomes in a pair is derived from an egg and a sperm, respectively. The normal karyotype is designated 46XX (for women) and 46XY (for men).
- The presence of any developmental defects in one of the partners;
- Chromosomal diseases in relatives;
- Spontaneous abortions, missed pregnancies or stillbirths;
- Presence in previously born children of developmental defects, including mental, and chromosomal diseases;
- Diagnosed infertility in marriage;
- Anomalies of sexual development;
- Prolonged contact with toxic substances or radiation;
- At least one of the partners is over 40 years old;
- Premature menopause;
- Sexual underdevelopment.
Karyotyping helps to identify changes in the number of individual somatic chromosomes (monosomy, trisomy and polysomy), for example, Down, Patau and Edwards diseases, as well as changes in the number of sex chromosomes that cause noticeable abnormalities in the psyche and mental development (Klinefelter syndrome, Shereshevsky-Turner disease).
For karyotyping, it is enough to get a blood sample from a vein. During the analysis, the nuclei of leukocytes are examined. If both parents are examined, blood samples are taken from each of them. In addition, karyotyping of fetal tissues is used in medical genetics. This can be done within the framework of invasive prenatal diagnostics or in the case of a missed pregnancy or miscarriage, in order to choose the optimal path to motherhood in the future.
Molecular genetic testing
To assess the risk of manifestation in an unborn child of pathologies such as cystic fibrosis, Duchenne muscular dystrophy or hemophilia, molecular genetic analysis is required - it allows you to identify the corresponding mutations of individual genes. The patient's venous blood is used for the analysis.
Fetal development can be abnormal, usually due to chromosomal abnormalities. The result can be not only a missed pregnancy, but also a stillbirth or death in the first year of a child's life. It is possible to identify such pathologies during pregnancy with the use of both non-invasive and invasive methods.
- woman's age over 35;
- chromosomal abnormalities and congenital developmental abnormalities diagnosed in previously born children;
- signs of chromosomal abnormalities revealed by ultrasound;
- balanced chromosomal rearrangement in one of the parents;
- significant risk of manifestation of pathology, identified by a blood test.
This study is carried out after the onset of pregnancy, at 10 weeks or more. To analyze the genome of the fetus, you do not need any manipulations affecting it: it is enough to do venous blood test. Specialists determine the presence of anomalies by studying fragments of the unborn child's DNA that are in the mother's blood. This can detect various chromosomal abnormalities, including Down syndrome. In addition, the sex of the fetus is determined for medical reasons.
In this type of study, a biopsy of the membranes of the fetus is carried out for further karyotyping of cells. The need for surgical intervention is a disadvantage of the method, however, the chances of miscarriage due to IPD are small: only 0.5%, while in normal conditions this figure is 15 to 25%. In a biopsy, the body of the fetus is not exposed to any influence - only a sample of umbilical cord blood or chorionic villi is examined.
IPD is performed in a hospital under the control of an ultrasound machine, without the use of anesthesia.
Chorion biopsy. It is carried out in the first trimester. Allows you to find out the presence of severe fetal pathologies at an early stage, as a result, termination of pregnancy will be the least dangerous for the patient. Among the disadvantages of chorionic biopsy is the possibility of a false positive result.
Placentobiopsy. It is carried out at 14-20 weeks of pregnancy. In this case, the cellular material of the placenta is examined.
Cordocentesis. It is carried out at 19-20 weeks of pregnancy, the material is umbilical cord blood. In addition to genetic research, it can be subjected to all possible types of tests (for hormones, infections, and so on). This IPD method is the most informative, but, unfortunately, it can only be applied at a later date.
IVF is not only a highly effective method of infertility treatment, but also an auxiliary method for diagnosing many hereditary diseases before pregnancy.
An additional procedure to assisted reproductive technologies is preimplantation genetic diagnostics of embryos - PGD or preimplantation genetic testing - PGT. The main advantage of this method is the absence of selective termination of pregnancy, as well as the high probability of having a child without a diagnosed genetic disease.
PGT is a check of an embryo obtained using in vitro fertilization (IVF) for the presence of genetic abnormalities before its implantation into the uterine cavity. In this way it is possible to select healthy embryos even before the transfer stage.
This technology has been used for almost 30 years, and every year its demand is growing along with the increase in the number of women who want to give birth after 35 years. The manipulation is performed only with IVF, does not pose a risk to the woman and does not affect future pregnancy. During natural conception, it is impossible to conduct such a study, but as an alternative method, prenatal testing is carried out.
However, if negative test results are received, the couple will be asked to terminate the pregnancy. Preimplantation diagnostics of hereditary diseases avoids this, since manipulation provides maximum information about the embryo, which makes it possible to select the highest quality cells.
The procedure is carried out not for everyone, only when it is known that the parents are either sick or are carriers of a monogenic disease. Thus, it is possible to determine whether the child will inherit chromosomal abnormalities from the parents or not.
Infertile couples are screened rather than tested, so there is little need for such testing.
The examination is performed to exclude mutations or unbalanced chromosomal rearrangements. Transfer of undiagnosed blastocysts is not performed.
More than 40 monogenic genetic diseases are currently known. The most common are:
- Cystic fibrosis
- Spinal muscular atrophy
- Sensorineural hearing loss
- Adrenogenital syndrome
- Martin Bell Syndrome (fragile X chromosome)
- Beta galactosemia
Types of PGD
PGS - preimplantation genetic screening.
PGT - Preimplantation Genetic Testing - is the same as diagnostics. Diagnostics means FISH analysis for a certain number of chromosomes, and testing means CGH / NGS analysis with diagnostics of all chromosomal pairs.
Several types of PGT studies:
PGT-A - aneuploidy analysis reveals extra or missing chromosomes in the cell nucleus, that is, it is aimed at selecting blastocysts with a normal number of chromosomes. It allows you to quickly get the desired pregnancy (increases the chances up to 70-75%) and avoid the transfer of aneuploids, for example, with Down's syndrome.
PGT-M - testing for the presence of hereditary diseases. Diagnostics is carried out only to identify a specific pathology, therefore, additional diagnostics of parents is necessary to clarify in which part of the gene the mutation could occur. This type of research guarantees the absence of a specific defect, but does not guarantee the absence of other abnormalities in the genome.
PGT-SR - testing for the presence of specific rearrangements in their DNA, which can also lead to various diseases and be inherited.
To determine which diagnostic method will be most effective for your couple, you need to make an appointment with a geneticist.
Indications for conducting diagnostics
Preimplantation screening is recommended for couples at risk of passing on known chromosomal abnormalities to their children. The presence of indications for the implementation of PGD is initially determined by the fertility doctor, and the decision on the appropriateness of such a study is made by the geneticist. It can be carried out in the protocol of in vitro fertilization at your own expense. There are no contraindications for conducting.
- Couples with X-linked disorders (risk for the child 25%, half of them are males);
- Couples with chromosomal translocations that can cause implantation failure, miscarriage, or mental or physical disorders in children;
- Carriers of autosomal recessive disorders (risk for the child is 25%);
- Carriers of autosomal dominant disorders (risk for the child is 50%).
- Examples of common diseases include:
- Cystic fibrosis,
- Tay-Sachs disease
- Gaucher disease
- Retinitis pigmentosa,
- Sickle cell anemia,
- Fanconi anemia,
- Duchenne myodystrophy,
- Baker's muscular dystrophy,
- Polycystic kidney disease,
- Alzheimer's disease,
- Familial breast cancer
- Familial adenomatous polyposis,
- Familial carcinoid syndrome,
- Chorea of Huntington,
- Fragile X syndrome,
- Spinocerebral ataxia.
Should you do PGD?
Preimplantation testing may be required in the following cases:
- Age of partners - women from 35 years and men from 40;
- Miscarriage, repeated spontaneous abortions, missed pregnancies;
- History of unsuccessful IVF attempts (two or more);
- Genetic abnormalities have been previously identified in children or in the fetus;
- Male infertility reasons;
- Severe disorders of spermatogenesis;
- Parents have hereditary diseases that have a gender identity;
- The presence of a high risk of having a child with a monogenic disease (for example, with cystic fibrosis, spinal muscular atrophy, phenylketonuria and other pathologies);
- Possibility of Rh-conflict between mother and unborn child.
At the request of couples, you can do PGD of the embryo for:
- Gender determination
- Human Leukocyte Antigen (HLA) Compatibility Testing
- Exclusion of late-onset diseases (Alzheimer's disease, breast cancer)
- Excluding non-life threatening diseases
- Testing for hereditary predispositions
Couples who cannot get pregnant naturally are consulted by a geneticist. A specialist, if necessary, prescribes tests to determine the karyotype and gives individual recommendations regarding the need for PGD before transfer.
A preimplantation examination is recommended for couples at risk of passing on known chromosomal abnormalities to their children. The presence of indications for the implementation of PGD is initially determined by the fertility doctor, and the decision on the appropriateness of such a study is made by the geneticist. It can be carried out in the protocol of in vitro fertilization at your own expense. There are no contraindications for conducting.
Where does PGD begin?
Preparation for the IVF treatment cycle with PGD practically does not differ from the usual IVF treatment cycle:
- Woman is taking hormonal drugs to stimulate superovulation;
- Follicles are collected using a puncture through the vagina;
- Fertilization (IVF / ICSI) of eggs with sperm is carried out in embryological laboratory;
- Cultivation in special conditions;
- Careful selection, preference is given to normally dividing and morphologically complete embryos;
- Laboratory biopsy of the trophectoderm on the 5-6th day of cultivation (using a mechanical or laser method), fixation of the blastomere. Previously, PGD was carried out on the 3rd day of cultivation, however, biopsy at this stage significantly reduces the viability of the embryos. At the same time, the collection of cells on the 5th day after fertilization does not affect their further development.
- Cryopreservation after biopsy of the trophectoderm;
- Genetic test of fixed cells and obtaining results;
- Embryo transfer into the uterine cavity of embryos without genetic defects;
- After two weeks, the standard procedure for diagnosing pregnancy.
The associated risks and results of the study depend on the skills and qualifications of the embryologist performing the biopsy. This procedure belongs to high-tech and expensive research. Therefore, it is preferable to contact large specialized centers immediately with the possibility of carrying out cytogenetic testing.
After two days, specialists evaluate the results and compare them with the development of the corresponding embryo.
If deviations are found in it, this means a mandatory refusal to transfer. Only those embryos that have not been found to have pathologies are transferred into the uterine cavity. If the study was carried out with the aim of giving birth to a child of a certain sex (only according to indications) or to prevent Rh-conflict, then the corresponding embryos are implanted.
The PGD scheme is used for a specific case. If the disorder is inherited from a woman, then only polar bodies can be tested. It is also possible to test only blastomeres. Alternatively, a sequential study of polar bodies, then blastocysts, can be carried out. Which scheme will be used is determined when consulting a geneticist.
Trophectoderm: what is it?
Currently, it is preferred to biopsy the trophectoderm rather than taking samples of not yet differentiated embryonic tissue. This approach allows to increase the viability of embryos and minimize the harm from PGD.
The development of the embryo occurs according to certain patterns. After fertilization, a zygote is formed, which begins to actively divide, and the number of germ cells grows exponentially. The duration of this stage is up to 3-4 days. Then the cells are divided into 2 portions: external and internal. In the thickness of the embryonic cell mass, an eccentric internal cavity is formed - the blastocoel, and those who have reached this stage are called blastomeres. Trophectoderm is the outer layer of the blastomere. It is the precursor of extraembryonic tissues required for normal embryo implantation and subsequent placenta formation. And the inner cell mass of the blastomere gives rise to the embryo itself.
The outer and inner sheets of the blastocyst have almost identical gene and chromosomal composition; therefore, the information content of the analysis of the trophectoderm is as high as in the study at earlier stages of embryo development.
On what day is the diagnostics done?
At the stage of preparation for IVF, couples often have questions: how long does it take to do PGD and when should the embryo transfer take place. In many clinics, preimplantation genetic diagnostics of embryos is performed on the 3rd day. After fertilization of oocytes with sperm in an embryological laboratory, the embryo develops - the cells divide. On the third day, the number of cells reaches 6-8, which allows genetic analysis of the embryo without damage to the further development of the embryo.
On this day, a biopsy takes place, that is, the extraction of one blastomere using special microinstruments. With the standard protocol without cryopreservation, the study is performed in a short time, since the embryo cannot continue to develop outside the mother's body beyond the blastocyst stage (5th day after fertilization). While genetic diagnostics is being performed, embryos continue to develop in the appropriate culture medium, after which they are transferred into the uterine cavity on the 5th day of development. By this time, the embryo reaches the blastocyst stage. An alternative approach is PGD in a cryocycle. In this case, a biopsy is performed on the 5th day of development, and then the embryos are cryopreserved. If in a protocol without verification, embryologists have no more than 48 hours left for analysis, then in a cryocycle, a specialist conducts genetic diagnostics within a month.
The practice of a separated cycle, in comparison with the standard protocol without cryopreservation, is characterized by: a lower risk of hyperstimulation, more material and time for analysis, as well as a biopsy procedure that is less traumatic for the embryo. Depending on the type of IVF protocol, specialists individually choose the timing of the PGD.
PGD methods used
Each method has its own advantages and disadvantages. The main differences are related to the chromosome number, the accuracy of the result and the cost of the procedure.
Multicolor FISH - fluorescent hybridization is used for numerical and structural chromosomal abnormalities. With the help of FISH hybridization, only a part of chromosomes is examined: three, five or seven chromosomes, most often chromosomes - 13, 18, 21, X and Y. Since some chromosomes remain unverified, this method does not guarantee high accuracy of the result, but reveals common genetic diseases.
Comparative genomic hybridization on CGH / aCGH microarrays (comparative genomic hybridization) tests all 23 chromosomes simultaneously, therefore, it provides high efficiency in culling embryos with aneuploidy. The cost of CGH is higher than FISH.
In the study of monogenic diseases, PCR-based methods are used. The polymerase chain reaction repeatedly and selectively copies a specific genomic segment that contains the gene for further research.
NGS - next-generation sequencing is a method for determining the sequence of DNA nucleotides, a method for decoding the human genome. It is currently the most common method for assessing the genetic status of an embryo. It detects gender-related gene changes, hereditary and embryonic mosaic mutations. With the help of sequencing, it is possible to avoid many mistakes in diagnostics, since the method has a very high sensitivity and specificity.
Embryo PGD: pros and cons
Today, such an examination is an alternative way to reduce the likelihood of pregnancy with a sick fetus and the birth of a child with a genetic disease. It should be borne in mind that PGD cannot be a complete substitute for prenatal diagnostics.
- Selection and transfer to the uterus of only those embryos that do not have chromosomal abnormalities
- Reducing the risk of spontaneous abortion in early pregnancy
- Reducing the risk of having a baby with certain defects
- Reducing the risk of miscarriage (approximately by 2 times)
- Reducing the risk of multiple pregnancies (by about 2 times)
- Increased chance of successful implantation (by about 10%)
- Increase the chances of a successful childbirth (by about 15-20%).
It is worth emphasizing that when women in the older age group (40+) decide to conceive a child using IVF, then the diagnostics of genetic disorders is the most reasonable approach to increase the chances of healthy offspring.
- possibility of accidental damage to the embryo (<1%)
- errors (up to 10%)
- the probability of wrong diagnosing of an embryo with pathology as normal (3.5%)
- probability of a wrong diagnosing of a normal embryo as pathogenic (10%)
- cancellation of the procedure due to the fact that pathology was found in all embryos (up to 20%)
The main disadvantages of the method:
- Possibility of damage to the embryo caused by insufficient qualification of the doctor (less than 1%).
- Obtaining false-negative and false-positive results in the presence of cell mosaicism in the embryo (a condition in which cells with 2 or more karyotype variants are present in the body, while, most often, one pool of cells contains a normal karyotype, and the others - "pathological"). When using a modern cytogenetic test, this risk is significantly reduced (no more than 10%).
- The duration of the testing leads to the postponement of the moment of embryo transfer and the use of cryopreservation. However, embryologists argue that the procedure should be followed by vitrification. This not only makes it possible to carry out thorough testing, but also to carry out the transfer in the natural cycle, which will significantly reduce the load on the female body and create conditions as close as possible to natural fertilization.
- The high cost of the technique. Its implementation significantly increases the cost of the IVF protocol.
- The need to undergo the IVF treatment cycle for parents with genetic diseases.
PGD is a complex manipulation of the removal of several cells from the blastomere, which must be performed by a specialist who has undergone special training and has experience in carrying out such studies. All over the world, the possibility of such a survey is limited by its cost.
Prices in St. Petersburg are approximately the same in all clinics, since the testing itself is performed by a limited number of laboratories that have the necessary equipment and trained personnel. The difference between clinics is in the availability of embryologists who can perform blastomere biopsy.