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Through different examinations, it is possible to assess very accurately the risk of a fetus suffering from this genetic disease. The explanations of Dr. Carine Abel, geneticist at the prenatal diagnosis center of the maternity hospital Croix Rousse in Lyon.
What is trisomy 21?
- Each of the cells of the organism contains 46 chromosomes, classified in 23 pairs: thus, we have 2 chromosomes n ° 1, 2 chromosomes n ° 2, 2 chromosomes n ° 3, etc. A person with trisomy 21 has 3 chromosomes # 21 instead of two. This excess genetic material is responsible for morphological abnormalities and intellectual disability.
- In this classic picture may be added cardiac and digestive malformations including behavioral disorders with autistic traits that are likely to make the prognosis heavier.
- It happens also, but rarely, that the 3rd chromosome 21 is not present in all cells of the body but only in some cells. This is known as trisomy 21 mosaic: the disease can be mitigated.
Is trisomy screening mandatory?
- The health professional who accompanies you during your pregnancy is obliged to offer you this screening. This is a provision of the law of bioethics.
- But you have every freedom to accept it or to refuse it.
What is the first phase of this screening?
It occurs during the first trimester of pregnancy, from 11 weeks of amenorrhea and before 13 weeks of amenorrhea and 6 days.
This is a combined screening based on three elements: certain ultrasound measurements by a sonographer with a specific approval, the determination of serum markers in the blood of the mother and the age of the latter.
- Ultrasound measurements Nuchal translucency (thickness of the fetal neck) and cranio-caudal length (the length between the crown of the skull and the coccyx). Thanks to statistical curves, it is known that a certain cranio-caudal length must correspond to a given thickness of the nuchal translucency. If the nuchal translucency is too thick compared to the cranio-caudal length, the risk of trisomy 21 is increased.
- Serum markers are proteins (the hCG beta protein and protein A) made first by the trophoblast and then by the placenta - and circulating in the mother's blood. If the beta hCG protein is too high compared to normal and the protein A too low, it increases the risk of trisomy 21.
- The age of the mother: the risk of trisomy 21 increases with maternal age. For example, if the risk is 1/1250 for a 25-year-old woman, it rises to 1/300 for a 35-year-old woman.
Combined screening consists of using computer software integrating the results of the ultrasound, those of the maternal blood test and the age of the mother. This gives a figure reflecting the level of risk of having a baby with Down syndrome (very low, intermediate or high). Only laboratories that have received a specific approval are authorized to make these calculations.
At this stage of screening, it is the assessment of a risk and not a definitive diagnosis. This one can only be posed after realization of the karyotype of the baby, that is to say a representation of the 23 pairs of chromosomes present in each of its cells.
What happens when the risk is very low?
- If the risk is less than 1/1000, it is considered that the probability of the child having Down syndrome is very low. Pregnancy surveillance is continuing normally.
- This risk assessment may be reviewed later, for example if the 2nd trimester ultrasound reveals a cardiac malformation. Further examinations will then be necessary.
What happens if the risk is intermediate?
- If the risk is between 1/51 and 1/1000, it is suggested to the mother to go further in the screening to better identify the risk.
- We then realize a PNID (non-invasive prenatal screening) performed from a simple blood test at the mother, so without any risk for pregnancy. It is 100% reimbursed by Social Security.
- This examination consists of studying the placenta DNA fragments that circulate freely in the maternal blood and to detect a possible overrepresentation of fragments from chromosomes 21. If this overrepresentation is highlighted, the PNI is positive. If no overrepresentation is detected, the PNI is negative.
- In the case of negative DPNI, a trisomy 21 fetal is very unlikely (even if there is a very slight risk of false negative of the order of 0.4%).
- In the case of positive PNID, it is highly probable (more than 99%) that the fetus carries a trisomy 21. It is then proposed to the parents to confirm the diagnosis by amniocentesis (collection of amniotic fluid containing fetal cells ) or choriocentesis (removal of placenta cells) to establish the karyotype of the baby.
What happens if the risk is high?
- If the risk established by the combined screening is higher than 1/50, the medical team generally proposes directly a fetal sample (amniocentesis or choriocentesis), without going through the phase of the DPNI (non-invasive prenatal screening).
- Many mothers still prefer PNIP upstream to avoid unnecessary exposure to the risk of miscarriage caused by these two sampling techniques (0.5% for amniocentesis, 0.7% for choriocentesis). With a 42-year-old mother-to-be, for example, combined screening may be pessimistic in the risk assessment because of the "weight" of age in the calculations, while the baby is not Down's syndrome.
If the diagnosis of trisomy 21 is confirmed, how are the parents accompanied?
- Prenatal Diagnostic Center Geneticist Receives Parents and explain to them what this genetic disease is, the implications for the child. He can advise them to meet parents of Down syndrome children in associations to learn about the daily care of this disability.
- It also gives them all the information on how the medical termination of pregnancy (GMI) would take place if they move towards this decision (this is the case for about 95% of parents). Up to 24 weeks of amenorrhea, childbirth is caused and the mother gives birth to an unsavory baby. After 24 weeks, the heartbeat of the baby in utero must be stopped, before giving birth: the gesture is a little heavier.
- Whatever their decision, parents may be accompanied by a psychologist.