What are prenatal tests and why are they done?

Currently, there are several prenatal tests that allow early detection of abnormalities or diseases. It is important to note that some of them pose significant risks.

Last update: June 16, 2022

Prenatal tests are the main means of control and follow-up during a pregnancy. They allow to monitor both the health of the mother and that of the baby. The goal is to ensure that the pregnancy proceeds normally.

On the other hand, prenatal tests They are used to detect any abnormality in the baby early.. This allows the pertinent measures to be taken to reduce its impact and avoid subsequent complications.

At present, it is possible to carry out prenatal tests of various types. Some of them are directed at the mother and others focus on the fetus. Sometimes they detect; others diagnose; and, of course, discard possible conditions or problems.

What are prenatal tests

Prenatal tests are essential procedures for the care of the baby and the mother.

Prenatal tests are procedures that are carried out during the three trimesters of pregnancy. The central purpose is to confirm that everything is going well and, if not, to prevent future complications.

The goals of prenatal testing are three:

  • Establish if there are health problems in the mother that may affect the baby.
  • Determine the basic characteristics of the baby and the pregnancy, such as size, gender, location in the uterus, etc.
  • Detect possible health problems, genetic problems or birth defects in the baby.

In general terms, prenatal tests are divided into three large groups:

  • Prenatal tests for the mother. It is a group of blood tests that are performed on the mother.
  • Screening or screening tests. They reveal the possibility that there is a problem in the fetus.
  • diagnostic tests. They confirm if the fetus has a specific problem.

Prenatal tests for the mother

As already noted, they refer to various blood tests. They are done to determine if there are any health conditions that could harm the baby. These procedures generally include the following:

  • complete blood count. It helps detect difficulties such as anemia, infections or coagulation problems.
  • blood group and Rh. If the mother is Rh negative and the fetus is positive, her body may begin to make antibodies against her child’s blood.
  • Hepatitis B and C. The hepatitis virus can be transmitted from mother to fetus. Usually only Hepatitis B is tested, but if certain risk factors are present, Hepatitis C is also done.
  • Rubella. If a woman contracts this viral disease during pregnancy, the baby is at risk of serious birth defects. The test detects if there are antibodies for this disease or not.
  • Sexually transmitted diseases. Some of these diseases cause miscarriage or are passed to the baby at birth. They can cause blindness, breathing problems, and other complications.

Prenatal tests to detect or diagnose abnormalities

This type of prenatal testing advance to establish possible diseases, genetic problemsabnormalities or defects in the baby. Some are not carried out routinely, but only when a risk factor is present or there is a well-founded suspicion that something is wrong. There are several types of these prenatal tests, as we will see below.


It is one of the prenatal tests that every pregnant mother should have. In principle, it is carried out to locate the placenta, establish the gestation period, find out the sex of the baby, etc.

From week 12, it is also used to detect structural congenital malformations that they are obvious. Likewise, from the second trimester it helps to detect factors that increase the risk of chromosomal alterations in the fetus.

Screening tests for the first trimester

Usually, are performed to estimate the risk of Down syndrome or other chromosomal abnormalities. It usually includes two tests: blood tests and ultrasound.

The conjunction of the blood test and the ultrasound yields early results. If these are abnormal, other tests will need to be done. to confirm the presence of the anomaly.

Screening tests in the second trimester

It consists, in principle, of a blood test to measure levels of a protein produced by the fetus known as alpha-fetoprotein. When the level of this is very high, there is a risk of problems such as the following:

  • Fetus with neural tube defect in the brain or spina bifida.
  • Congenital malformations in the abdominal wall of the fetus.
  • High risk of miscarriage.
  • Fetal death.

Also, these prenatal tests measure the levels of other substances such as estriol, human chorionic gonadotropin, and inhibin A, which affect congenital malformations. If it is considered necessary, imaging tests such as ultrasounds are also done to observe what is happening in more detail.

On ultrasound, the baby’s nuchal translucency and cranio-coccyx length are observed. If the result was not what was expected, the fetal DNA test in maternal blood could be used, one of the most reliable and least risky tests for pregnancy.

Fetal ultrasounds

This type of prenatal testing are performed when a woman has an abnormal blood test result, like those already mentioned. Also, if there is a family history of birth defects, such as cleft lip, congenital heart disease, etc.

The results of this type of procedure are suggestive, but not conclusive.. Usually, after the fetal ultrasound, another prenatal test is carried out to confirm suspicions of a possible anomaly.


This is one of the most used prenatal tests to detect abnormalities before birth. It is generally offered to mothers over the age of 35. However, any pregnant woman can have this procedure. It should be done after the 15th week of gestation.

It consists of extracting a sample of amniotic fluid and analyzing it in the laboratory.. There, a culture is carried out and the chromosomes are analyzed, as well as the level of alpha-fetoprotein. The results allow detecting the risk of a defect in the brain or spinal cord. The test has certain risks.

chorionic villus biopsy

This is one of the prenatal tests that is applied to diagnose disorders in the fetus. It is carried out, almost always, between the 10th and 12th week of pregnancy. It consists of taking a sample of the chorionic villi, which are part of the placenta.

The sample can be obtained through the cervix or the abdominal wall. The goals and results are similar to those of amniocentesis, but has the advantage that the diagnosis can be obtained sooner. After the test, miscarriage occurs in 1 in 500 cases.

Percutaneous umbilical blood sampling

It consists of extracting a sample of blood from the fetus, from the umbilical cord. Currently, this test is very rare, as it ends in miscarriage in 1 in 100 mothers who undergo it. It is usually only done if there is a well-founded suspicion of anemia in the fetus. Also, it allows detecting chromosomal abnormalities.

Pre-Implantation Genetic Testing

If fertilization has been carried out in vitro, a diagnosis of genetic disorders is likely to be made in the embryo before it is implanted in the womb. Typically, this type of prenatal testing is only done for couples at high risk of having babies with genetic abnormalities.


Cardiotocography allows evaluating the heart rate of the fetus and its movements.

This is one of the prenatal tests that allows evaluation of the heart rate and movements of the fetus. It can be of three types:

  • Resting. Neither the movement of the fetus nor the contractions of the uterus are stimulated.
  • with contractions. Mild contractions are induced in the uterus to see the response of the fetus. This allows you to anticipate how you will respond to delivery. If that response is not favorable, a cesarean section may be required.
  • biophysical profile. Assesses the fetus’s breathing, movements, and amniotic fluid volume. These variables allow us to know if an early delivery is necessary.

Talk to your doctor about the prenatal tests you should have

Prenatal tests become more relevant in adolescent mothers or those over 35 years of age. Also, in those who have had babies with birth defects or premature, or have a health problem such as hypertension, lupus, diabetes, etc.

Every mother should talk to her doctor and be well informed about the advisability of having prenatal tests. It is very important that you know the advantages and disadvantages of each of them. In the end, it is the mother who must decide if she practices them or not.

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