If we ask a couple what worries them most about pregnancy, the answer is usually "That the child is well". During pregnancy we are going to carry out several tests to check that everything is going well and that our baby does not have any problems. One of the possibilities is to find out if you are not a carrier of chromosomal abnormalitiesCurrently there are several ways to verify that this is not our baby's problem.
Normally with the results of the first ultrasound a blood draw is performed, in which the values of certain hormones are determined and with other data, such as the mother's age, weight and whether or not she is a smoker, the "Triple screening", with which they will tell us the statistical risk that our baby is a carrier of a chromosomal abnormality such as Down syndrome or Edwads syndrome. This is a very reliable test, around 95% accurate. It is considered high risk if it is greater than or equal to 1/250 for either of the two chromosomal abnormalities. Until now when this was intermediate risk or there was any other circumstance that created doubts about the result, a invasive test, such as the amniocentesis(perform a puncture in the mother's abdomen, accessing the bag that protects the baby to extract a sample of amniotic fluid and analyze it) or chorionic biopsy(access the posterior area of the placenta to extract a small sample for the same purpose as amnicehesis), increasing risk being able to break the bag or get an infection and lose the pregnancy.
Since 1997, it is known that when a woman is pregnant, her blood can detect, in addition to its DNA, the DNA of the fetus, so a few years ago the possibility of developing a test to detect the baby's karyotype began to be investigated undone invasive tests and some time ago the fetal DNA test in maternal blood.
What does it consist of
It consists of carrying out a blood test to the mother to detect fetal DNA. DNA free from baby cells circulating in the maternal blood. DNA is the genetic component, which determines not only the inheritance of the baby, but also if it is a carrier or suffers from any chromosomal disease or sex, mainly the presence of trisomies 21 (Down syndrome), 18 (Edwards syndrome) and 13 (Patau syndrome) or numerical alterations of the sex chromosomes. The predictive value of the test is 99,9% for Down syndrome, and slightly less for Edwards syndrome and Patau syndrome. This means that the precision is significantly better than the previous methods. Only if the results are positive they have to be confirmed with chorionic biopsy or amniocentesis. Unlike Amniocentesis or Corial biopsy, the fetal DNA test does not carry any risk to the baby (or the mother), since only one blood is drawn from the mother's arm.
When to do it
When triple screening dont have a conclusive result, the parents be carriers of some chromosomal disease, appear strange data on ultrasound or whenever parents wish it. It is important to be well informed in case of twin pregnancies or in those achieved with egg donation, in these cases loses sensitivity and before carrying out it, it is convenient to be clear about the possibilities it offers us and its sensitivity in these cases.
The test can be carried out from the week 10 pregnancy so that the results are reliable.
How is it done and how long does the result take?
It involves a blood draw to the mother. It can be done at any time of the day and does not need preparation of no type.
Results take between 8 and 10 days.
What if the result is positive?
In the event of a positive result for a certain chromosomal alteration, the confirmation by invasive prenatal diagnosis (amniocentesis or chorionic biopsy), since is not considered a definitive diagnostic test. Although the false positive rate is very low, it is essential to confirmation with one of these techniques.
Where can I do it? Does the Health Service of my Autonomous Community cover the cost of the test?
There are several laboratories that have the test privately. Its cost are about € 700. There's no need no requirement To do so, if the couple decides that they want to supplement the tests carried out in the follow-up of their pregnancy, although until that moment there is no cause for alarm, they can do so privately. In the Community of Madrid, the monitoring protocol for pregnant women at Hospital Clínico San Carlos has included the free fetal DNA test in maternal blood in cases where triple screening or ultrasound has a doubtful result .