The most reliable method of screening for Down syndrome during pregnancy and a few myths and facts about Down syndrome diagnosis during pregnancy.
A person or child with Down syndrome has an extra chromosome in their genetic makeup, resulting in characteristic physical and mental changes. The degree of these changes varies from minimal, when the person can take care of himself, to maximum, when he needs help and is profoundly disabled.
During pregnancy, how can you tell if the baby you are carrying has a chromosomal abnormality?
It is possible to accurately determine only having the cellular material of the fetus - for this purpose it is necessary to perform a puncture of the uterus and to obtain either cells of amniotic fluid, in which skin scales with fetal skin cells are floating, or to obtain samples of the rudiment of the future placenta, which has a chromosomal set identical to the chromosomal set of the fetus. Such a procedure requires a specially trained specialist, means and carries a 1-3 percent chance of miscarriage.
That is why screening examinations are performed everywhere in the first and second trimesters, i.e. the probability of miscarriage is determined. In this case, a blood test of the mother and an ultrasound of the fetus are performed. The data are then summed up, and a special program assesses the degree of risk - high or low. For those who are found to be at high risk, it is recommended to perform the diagnostic tests I described above. There is another screening test, when fetal blood cells are found in the mother's blood and their chromosome set is examined. This is the NIPT, information about it is below.
It is important to remember that screening and diagnosis for Down syndrome is purely voluntary, but it is necessary to understand and imagine for yourself what you will do if the fetus is confirmed to have Down syndrome.
‼️ Below are a few misconceptions that exist among women.
❌ "My baby can't have Down syndrome because I'm young, healthy and have a healthy lifestyle."
♦️lifestyle is undoubtedly only beneficial to the fetus, but it does not affect the likelihood of SD in the fetus. It can happen in a young and healthy woman as well.
❌ "My child cannot have DM because my family members, as well as my husband's family members, have not had any cases of a child born with DM."
♦️ a child can have DM even if there have been no cases in your family or your husband's family.
❌ "I am not going to terminate the pregnancy even if my baby is found to have DM, so I will not be screened."
♦️ Even if you are not going to terminate the pregnancy, it is still recommended to undergo screening in order to find out whether the child has DM in combination with heart disease or any other pathology that requires delivery in a specialized institution.
It is important (!) to remember that the diagnostic accuracy of screening tests with a combination of maternal blood tests and ultrasound examination is 84-90% according to the UK National Health Service. A more accurate non-invasive method, but also screening (!), is the study of fetal cells detected in the mother's blood, NIPT (non-invasive perinatal test).
The test is based on the fact that placenta cells and their debris in small quantities are constantly circulating in the mother's blood. These cells have the same chromosome set as the fetus. Only venous blood of the pregnant woman is needed for the test, but not earlier than 10 weeks of pregnancy. The cost of the test depends on the country and the laboratory or clinic where you are served.
This test can detect abnormalities of chromosomes 21, 18 and 13, and additionally, the sex of the fetus. But it cannot detect neural tube abnormalities, heart defects and anterior abdominal wall defects, which are detected by ultrasound. Nor can it detect other genetic disorders. That's why a comprehensive exam is important!
How do you rate the results?
When you get the result, there can only be 2 options there - "positive" or "negative".
But it's not that simple here!
If the result is negative, there is no 100% guarantee here either. But the good news is that in 99% of cases it will indeed be negative. That is, out of 100 women who are negative, 99 will be truly negative, and one woman who is negative will still have a baby with Down syndrome. The error rate is 1%.
But with a positive result (confirming the syndrome) it is even more difficult. The interpretation of the result and its diagnostic value depends on whether the woman is in the risk group for Down syndrome.
The diagnostic value for a positive result, also called "positive predictive value", is 85% for high-risk women and 33% for low-risk women!!!!
For example, out of 100 women in the high-risk group with a positive NIPT result, 85 women will indeed have a fetus with DM. But 15 women in the same group with a positive result will not have Down syndrome in their fetus. In other words, the error is 15%.
In the 100 low-risk women with a positive result, only 33 of them will actually have a fetus with DM, and 67 women with a positive result will not have Down syndrome in their fetus. That is an error of 67%!
What to remember?
- NIPT is a reasonably accurate SCREENING method of diagnosing fetal DM, but it has its limitations and restrictions.
- If the test is negative, it does not rule out fetal DM.
- If the result of NIPT is positive, it does not confirm (!) the presence of DM in the fetus, and requires diagnostic tests - chorionic villus biopsy, amniocentesis, cordocentesis.
-NIPT alone is not enough (!!!) to diagnose neural tube pathology and other conditions. It is necessary to perform ultrasound examination and other tests.
As you can see, not everything is so simple with this NIPT.....