When we talk about a Prenatal screening o screening we are talking about a test that allows us to identify those pregnant women with a high risk of suffering a specific disorder, with the aim of last to initiate a more thorough investigation or take direct preventive action.
Among the general population of visually healthy pregnant women, those who are at higher risk of the fetus carrying a chromosomal disease, which could be recognized by a diagnostic test. The joint use of PAPP-A, free BHCG, as biochemical tests, TN (Nuchal Translucency) and maternal age, as screening for chromosomal diseases in the first trimester of gestation, is recommended.
En Women's Barcelona we made the prenatal screening for the detection of chromosomal abnormalities.
The incidence of chromosomal abnormalities is 4-5% of all pregnancies, being responsible for a high embryonic and fetal lethality. They constitute 1-2% of all congenital defects. They are of two types: autosomal (numerical and structural alterations) and sexual. They are diagnosed by invasive methods such as amniocentesis or chorionic biopsies.
Combined screening is a screening test performed in the first trimester of pregnancy in all pregnant women in order to detect those pregnancies with a higher risk of having a fetal chromosomal alteration.
It is a non-invasive test (without risk to the mother or the fetus), which is performed from a blood sample from the mother and a fetal ultrasound and is offered to all women.
The risk of chromosomopathy (alteration of chromosomes) is obtained by combining biochemical markers present in maternal blood and first trimester ultrasound. Screening offers a personalized statistical probability that the fetus may have a chromosomal defect (such as trisomy 21 and other abnormalities).
It is normally performed between week 8 and week 10. In this extraction, two biochemical values are analyzed in the mother's blood: the beta fraction of the pregnancy hormone (BHCG) and the value of the placental protein associated with pregnancy (PAPP -TO). Fasting is not necessary to carry out this extraction.
Ultrasound is performed between 11,3 and 13,6 weeks of gestation. It is one of the three ultrasounds recommended by the Spanish Society of Gynecology and Obstetrics (SEGO) and one of the most important throughout pregnancy. During this ultrasound, different aspects will be assessed:
- Gestation dating, measuring the length of the fetus (CRL).
- Early fetal anatomical study.
- Assessment of chromosomal disease markers: nuchal translucency (accumulation of fluid at the level of the fetal neck and present in all fetuses. International studies have shown that fetuses with Down syndrome tend to have a greater accumulation of fluid at the level of this space) and other secondary markers such as the presence of nasal bone, ductus venosus wave and tricuspid regurgitation.
For a correct calculation of the screening, it is necessary to take into account other factors of the pregnant woman: race, weight, eventual twin pregnancy, maternal pathologies (insulin dependent diabetes), maternal habits (smoking) or if the pregnancy has been achieved by techniques of Assisted reproduction. The introduction of these factors in the calculation of the screening allows making small corrections that improve the precision of the risk calculation.
Calculate it from risk
Usually a computer software calculates the risk index using all the entered maternal and fetal variables. The calculation of the risks of trisomy 21 (Down syndrome), 18 (Edwards syndrome) and 13 (Patau syndrome) will be made from the risk conferred by maternal age and modified according to the deviation of the ultrasound data and biochemicals introduced.
In the event of a high-risk result, an invasive test, chorionic biopsy, or amniocentesis will be offered to confirm or rule out the suspected diagnosis.
Currently, this test is considered of choice for all pregnant women, presenting a detection rate of trisomy 21 (Down syndrome) of around 90% with a false positive rate of 5% (cases that according to the test would be high risk when in fact there is no chromosomal disease).