Pregnancy and gestation are very delicate stages, since in this biological process the new organism begins to develop. This is why, from a medical point of view, it is important to know as much as possible about what is happening in the development of the foetus , in order to be able to intervene as soon as possible in the case of congenital diseases.

Amniocentesis is the procedure that doctors perform to obtain this early information in order to make an early diagnosis during pregnancy. Throughout this article we will review everything you need to know about this test: what is amniocentesis, what are its functions, how it is performed and what are the risks to take into account.

What is an amniocentesis?

We call amniocentesis a type of prenatal test in which through a medical procedure an early diagnosis of chromosomal diseases and fetal infections is made and which secondarily also helps us to know the sex of the baby before birth.

To understand how it works, we must first know that throughout the gestation stage the fetus is surrounded by a substance called amniotic fluid , whose composition includes fetal cells. From the observation of this fact, the scientific community applied to the clinical field has discovered that amniotic fluid is capable of giving us useful information about the health of the baby months before birth. Amniocentesis focuses on the analysis of this substance and its components.

When performing the amniocentesis, a small sample of amniotic fluid is obtained through the use of a needle that is introduced into the woman’s abdomen at the same time as an ultrasound scan is performed with which the process can be monitored. Secondly, the sample of amniotic fluid obtained is analysed in the laboratory, where the DNA of the foetus is studied to see if there are any genetic abnormalities in it.

When is it done?

This prenatal test is only offered to those women who are at significant risk of genetic disease. In most cases, the main reason for performing an amniocentesis is to find out if the fetus has any chromosomal or genetic abnormalities as can occur in Down syndrome. This diagnostic procedure is usually scheduled between 15 and 18 weeks of gestation .

Therefore, it is not always necessary to do it, in most cases it is only done in those pregnant women where the baby presents some risk of developing some genetic pathology. The reason why it is not done to all women is that it is a rather invasive test that carries a small risk of miscarriage .

Since amniocentesis is associated with certain risks, a complete anatomical ultrasound is performed before it is performed, in order to detect abnormalities in the baby. In cases where there is reason to suspect the existence of genetic or chromosomal disorders , the amniocentesis will be performed.

Functions of this test: what is it for?

The main cases in which an amniocentesis is required include

  • A family history of birth defects .
  • Abnormal ultrasound test results.
  • Women with pregnancies or children in which there are birth or gestation disorders .

Unfortunately, amniocentesis does not detect all possible birth defects. However, the ultrasound test, which is performed at the same time, can detect birth defects that cannot be reported in the amniocentesis such as cleft lip, heart defects, cleft palate or clubfoot.

However, the risk of some birth defects not being detected by either diagnostic test cannot be ruled out. In general, the main diseases detected by amniocentesis are

  • Muscular dystrophy.
  • Cystic Fibrosis.
  • Sickle cell disease .
  • Down syndrome.
  • Alterations in the neural tube , as occurs in spina bifida.
  • Tay-Sachs disease and related.

Finally, the accuracy of amniocentesis is about 99.4%, so even though it has certain dangers it is very useful in cases where there is a real suspicion of fetal anomaly.

How do doctors perform it?

After cleaning the area of the abdomen where the needle will be inserted with an antiseptic and administering a local anesthetic to relieve the pain of the prick, the medical team locates the position of the fetus and the placenta using an ultrasound scan. Guided by these images, a very fine needle is introduced through the mother’s abdominal wall , the wall of the uterus and the amniotic sac, trying to keep the tip away from the fetus.

A small amount of liquid is then extracted, about 20 ml, and this sample is sent to the laboratory where the analysis will be performed. In this space the fetal cells are separated from the rest of the elements present in the amniotic fluid.

These cells are cultivated, fixed and stained so that they can be observed correctly through the microscope. Thus, the chromosomes are examined for abnormalities .

As for the baby and its environment, the puncture seals and amniotic sac fluid regenerate over the next 24-48 hours. The mother should go home and rest for the rest of the day, avoiding physical exercise. Within a day, she can return to a normal life unless the doctor says otherwise.

The risks

Despite the fact that safety measures in medicine have made great progress in this area as well, amniocentesis always presents risks . The risk of miscarriage is the most notorious, although it occurs in only 1% of cases.

The possibility of premature birth, injuries and malformations in the fetus is also an aspect to consider.

Bibliographic references:

  • Carlson, L. M. & Vora, N. L. (2017). Prenatal Diagnosis: Screening and Diagnostic Tools. Obstetrics and Gynecology Clinics of North America, 44(2): 245-256.
  • Seeds, J. W. (2004). Diagnostic mid trimester amniocentesis: how safe? American Journal of Obstetrics and Gynecology, 191(2): 607-15.
  • Underwood, M. A, Gilbert, W. M, Sherman, M. P. (2005). “Amniotic Fluid: Not Just Fetal Urine Anymore. Journal of Perinatology. 25 (5): pp. 341 – 348.