VALENTINA GNETEKSKY, Ph.D., geneticist, head. Department of Prenatal Diagnostics of the Center for Family Planning and Reproduction
Modern medicine is able to exclude most of the congenital diseases before the baby is born. For this, modern methods of prenatal (prenatal) diagnostics have been developed, one of which is biochemical screening, that is, a blood test for specific proteins.
The word “screening” in translation from English means “screening.” With this method, chromosomal diseases can be “screened out” – for example, Down Syndromes (this technique was once developed to combat this ailment), Edwards, as well as the defects of the central nervous system and other serious disorders.
Prenatal screening is a whole program that assesses the likelihood of having a baby with genetic diseases for each woman. In addition to special blood tests, it includes ultrasound on the 10−13, 20−23 and 30−33 weeks of pregnancy.
If doctors find a high risk of problems, the expectant mother will also be asked to do an analysis of the chromosome set of the child – karyotyping.
Biochemical screening is carried out in 2 stages: at a period of 11−13 and 16−20 weeks of pregnancy. During the first study (it is also called a double test), doctors evaluate the blood content of a woman’s two proteins, which are produced by the placenta: plasma protein (PAP) and human chorionic gonadotropin (beta-hCG).
During the second analysis – a triple test – the level of three proteins is determined: alpha-fetoprotein (AFP), hCG and free estriol. Changes in the concentration of these substances may indicate a genetic disease in a child.
According to the double test, doctors calculate the risk of Down and Edwards syndromes, and based on the results of the triple, also the neural tube defects.
Before making a prediction, in addition to blood counts, doctors take into account a number of data that may affect the results of screening:
- gestational age;
- age of the future mother;
- her weight;
- bad habits (smoking);
- features of the course of pregnancy;
- ultrasound data.
Each of these factors is very important: for example, if a woman smokes, the level of beta-hCG in her analysis will change. And if she is over 35 years old, the risk of chromosomal abnormalities in a future baby increases already due to her mother’s age.
The results of the blood test must be considered together with the ultrasound data. Special attention is paid by experts to special indicators (markers), on which chromosomal abnormalities can be suspected.
In the first trimester, the most important of them are the thickness of the neck fold, or collar space (for a period of up to 14 weeks it should be no more than 2.5 mm) and the size of the bones of the nose. In the second trimester in the number of markers – a shortening of the femurs, especially the shape of the head, the expansion of the renal pelvis.
True, it must be remembered that each of these disorders can occur not only in chromosomal diseases, and this data is not a reason to draw final conclusions.
When all the information is collected, the risk of problems for each woman is calculated by a computer program. In Russia, a risk index of 1: 250 was adopted as a “starting point” – this means that 249 women with the same test results will give birth to healthy children, and one – a child with Down syndrome or other disorders.
If this figure is less (for example, 1: 200, 1: 100, 1: 50), the probability of problems is considered increased.
In order to make a more accurate prognosis and develop a further program of action, the expectant mother will be sent for consultation to genetics – this doctor can be found in major centers of obstetrics and gynecology and in specialized medical and genetic consultations. If the risk of disturbances in a child is high, the doctor may suggest the most accurate diagnostic methods to the mother – intrauterine devices (invasive).
At the same time, doctors can examine the baby’s chromosome set in placenta cells (they are obtained using a chorion biopsy — the future placenta — for a period of 10–12 weeks or a placenta at 13–18 weeks), baby cells contained in the amniotic fluid (amniocentesis in 16–20 weeks) or cord blood (cordocentesis at 21–23 weeks).
After examining the chromosomal set of the child, you can identify any genetic disease – not only Down syndrome and Edwards, but also dozens of other defects. The overwhelming majority of mothers, these studies help to dispel all anxiety: in 93% of women at “risk group” chromosomal analysis is normal.