Modern prenatal diagnosis is 2 types – non-invasive and invasive. In the first case we are talking about screening studies (“double” and “triple” tests).
A pregnant woman takes blood from a vein, and usually an ultrasound scan is also performed on the same day, the purpose of which is to assess the key parameters of the development of the organs and body parts of the future baby. “Double” test is done for a period of 11−13 weeks. The blood levels of hCG and plasma protein are determined. A “triple” test is performed for a period of 16–20 weeks.
In addition to hCG, the level of alpha-fetoprotein and free estriol is determined in the blood. However, according to the results of screening tests, doctors can only assume the degree of risk of genetic abnormalities.
If he is tall, the expectant mother is sent for a consultation to a geneticist who can offer invasive examination methods. They are considered much more accurate, because, depending on the gestational age, the chorion cells (embryonic embryonic germ layer) or the baby itself, which are in amniotic fluid or cord blood, are sent for analysis.
It is possible to get such a biomaterial only when “invading” into the uterine cavity, therefore, such studies are called invasive (from the Latin invasio – introduction, invasion, penetration inside). It is time to figure out how they are carried out and why.
The main risk during any type of invasive prenatal study is the threat of spontaneous abortion. All due to the fact that during the procedure may increase the tone of the uterus.
But, firstly, in future moms, to whom these manipulations were not performed, it is exactly the same and does not exceed 2%, and secondly, doctors know how to keep it to a minimum: 2 days before the study and 5 more days after the future Mom is recommended to take antispasmodic drugs that relax smooth muscles. With amniocentesis, there is also a risk of premature rupture of the amniotic fluid, because during this procedure, the integrity of the fetal bladder is disturbed.
To prevent this complication is impossible, but the risk itself is small – only 0.3%.
Considering a number of shortcomings, doctors recommend the procedure only if the probability of a serious illness in a baby exceeds the possibility of unfavorable scenarios. And, of course, speaking of the drawbacks, do not forget about the advantages: the vast majority of women invasive diagnosis helps to alleviate all anxiety: in 90% of cases, doctors’ fears are not confirmed.
The direction to conduct an invasive study are primarily those women who are at high risk. The likelihood of deviations from the genetic norm in the unborn child increases if the woman is 39 years old and more, if the family already has a baby with a chromosomal abnormality or its carrier is one of a family member, and also if the next of kin have monogenic diseases (eg, cystic fibrosis) .
This group also includes women who, before conception or early in pregnancy, took medications that affect the development of the embryo, or have had such infections as hepatitis, rubella, toxoplasmosis. There may be genetic or chromosomal abnormalities even if the pregnancy had spontaneously been interrupted at least 2 times before.
An invasive study may also be needed if the woman is not at risk, but the screening test made the doctors alert.
Free invasive diagnosis is carried out in the direction of the antenatal clinic, and only for medical reasons. But if the future mother wants to hedge, then she can undergo a similar study on her own initiative – for a fee.
There are only relative contraindications to invasive diagnosis, for example, infections of the genital tract, the presence of fibroids in the needle insertion path, etc. This means that the doctor will assess the situation in the complex in order to find the most optimal research method and time for it.
There are no strict prohibitions, and theoretically almost any woman can undergo such a diagnosis theoretically.
Both chorionic biopsy, amniocentesis, and cordocentesis are controlled by ultrasound, usually without anesthesia in the hospital for one day. In some cases, intravenous anesthesia is possible. After the procedure, the expectant mother is under the close supervision of doctors for another 1-3 hours.
If disturbing symptoms appear during this period (increased uterine tone, lower abdominal pain or discharge), hospitalization is required, although this course of events is taken very rarely.
After taking the puncture, the biomaterial is sent for cytogenetic and molecular genetic analysis. With the first one, the deviations in the chromosomal set are determined (Down syndrome is an extra 21st chromosome, Klinefelter is an extra X chromosome, Turner is the absence of an X chromosome in a future girl).
The second analysis is needed to find intrachromosomal defects – gene mutations that cause diseases such as hemophilia, phenylketonuria, Duchenne muscular dystrophy, cystic fibrosis (only about 50 severe genetic abnormalities are available in Russia). The blood taken during cordocentesis is also subjected to biochemical research.
Chorionic biopsy is performed on the period of 10−14 weeks of pregnancy. Chorion – the outer germinal envelope surrounding the embryo – is literally penetrated by a set of elongated vessels that grow into the wall of the uterus.
As the duration of pregnancy increases, they gradually turn into villi, through which the metabolism occurs. They are taken for analysis during the puncture. Since the genomes of the cells of the chorion and the embryo overlap, the vorsins of the chorion allow us to judge the chromosomal set of the future baby.
For example, you can quite accurately determine Down’s syndrome, Edwards, Patau and other chromosomal diseases, accompanied by gross defects and mental retardation. At the same time, due to incomplete coincidence of genomes, false-positive or false-negative results are possible.
The accuracy of the analysis is 99%.
The material for the study is taken using a needle inserted into the uterine cavity through the abdominal wall. The risk of pregnancy complications in this procedure is 1-2%.
If the chorionic villi did not have a biopsy, then at 14–16 weeks a woman is referred for a placental cell biopsy — the placentocentesis. Manipulation is done in the same way.
If the chorion biopsy could not be completed on time, the woman will be referred for amniocentesis, a puncture of the amniotic fluid. This study is conducted at a period of 16−19 weeks of pregnancy.
The fetal bladder, or amnion, is the inner germinal membrane surrounding the future baby. Its cavity is gradually filled with the so-called amniotic, or amniotic fluid. It contains fluff hair and epithelial cells of the unborn child, and it is possible to determine the presence of chromosomal or gene diseases.
And since the metabolic and metabolic products of the baby enter the amniotic fluid, they can be used to judge the degree of rhesus conflict, oxygen starvation or the maturity of the lungs.
During the procedure, a needle is inserted into the uterine cavity through the anterior abdominal wall and 15 ml of amniotic fluid are collected. Results are known in 2–3 weeks. The probability of pregnancy complications is less than with chorionic biopsy (less than 1%), since the life support systems of the baby are not affected.
There is also a low risk (also less than 1%) of respiratory disorders in the newborn. The accuracy of the results is 99.4% – higher than with the chorion biopsy, since the baby’s cells are examined directly.
Cordocentesis, umbilical cord puncture, is performed on the 19th – 21st week of pregnancy. This is the most informative method of prenatal diagnosis, because doctors can get the blood of the baby.
Such a biomaterial, in addition to determining chromosomal abnormalities, allows one to perform almost all tests that can be done from a biochemical blood test (hormonal examination, evaluation of biochemical parameters and immunological status, etc.).
In order to detect chromosomal abnormalities, cordocentesis is performed if the woman has not had time to do amniocentesis; if the results of an amniocentesis cannot be interpreted unequivocally; if on the 19th-21st week ultrasound revealed malformations. This manipulation is also prescribed in a number of other cases, for example, to check whether the baby has become infected by a serious infectious or sexually transmitted disease from the mother (toxoplasmosis, rubella, syphilis, etc.).
During the procedure, a needle is inserted into the baby’s umbilical vein through the anterior abdominal wall and the wall of the uterus. Results are known after 4–5 days.
Their accuracy is almost 100%. The frequency of complications does not exceed 2%.
A 100% guarantee does not provide any invasive method of prenatal diagnosis. The accuracy of the results is influenced by two key factors. For cytogenetic analysis, it is required to obtain cells at a certain stage of division, which is called mitosis.
Otherwise, count the number of chromosomes and notice the changes impossible. To force cells to divide, they are placed in special nutrient media. But during cultivation, they can mutate.
The slower the division, the more time it takes to grow it, which means that the probability of mutations increases. It is for this reason that cordocentesis gives the smallest percentage of errors: blood lymphocytes divide in just 72 hours.
However, speed is not the only success factor. The origin of the biomaterial is also important; therefore, amniocentesis is considered the second in terms of accuracy. Although the epithelial cells obtained during the puncture of the fetal bladder multiply 2–3 weeks, the probability of errors is lower than with a chorionic biopsy, because they belong directly to the baby.
The waiting time for response to amniocentesis is also delayed because 15 ml of amniotic fluid are taken for the test, and in such a volume of baby cells, it is usually very short and it takes more time to reproduce. Chorion biopsy (or placenta) gives the largest percentage of errors.
One of the reasons is that, although the cells of these tissues multiply rapidly, their genomes with the genome of the future baby coincide only partially.
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