Genetic Testing and Screening

Genetic Testing And Screening Options During Pregnancy

According to the Centers for Disease Control and Prevention in the United States, there are over 3.9 million births every year in the nation. While a majority of these babies are born healthy, there are those who are born with congenital disabilities.

According to the CDC, at least 1 in every 33 babies born in the nation is born with a congenital disability or birth defect. Many of these defects can arise at any time during the pregnancy with most of them occurring in the first trimester. Further, many of these congenital disabilities are genetic meaning that they result from changes in an individual’s genes or chromosomes.

Some factors increase the risk of genetic disorders and birth defects in a baby. Perhaps one of the biggest risk factors is the family and personal history of congenital disabilities or genetic disorders of the mother or the father. Such history may lead to the baby inheriting the genetic disorder from the parents.

Other risk factors for birth defects and genetic disorders in babies include; ethnic background and even the age of the pregnant woman. For example, women who get pregnant when they are 37 years old or above have a higher risk of getting a baby with a genetic disorder.

Sometimes, however, genetic disorders in pregnancy are genetic accidents that happen randomly. For example, a fetus may have missing chromosomes (something known as a monosomy) or even have extra chromosomes (something referred to as a trisomy). So the absence of risk factors does not necessarily mean no chance of having a baby with a birth defect.

Thanks to advancements in the medical field, it is now possible for pregnant women and even those planning for pregnancies to find out whether there is a risk of the baby having a birth defect or inheriting a disorder. This is done through genetic testing and genetic screening during or even before pregnancy.

It is important to note that while these tests are offered to all pregnant women, they are optional. It is therefore up to the woman (and her spouse) to decide whether or not to have the genetic screening and genetic tests done.

Common Birth Defects and Genetic Disorders

It is possible to use genetic screening and genetic testing to determine the risk for some possible birth defects and genetic disorders. These include;

1. Chromosome Abnormalities

(Source: Ligra Istanbul)

These include conditions such as Down Syndrome (trisomy 21), trisomy 18 ( Edwards Syndrome) and trisomy 13 (Patau Syndrome) among others. Chromosomes are the chemicals within the body cells that are used to carry DNA and genetic information. Trisomy 21 is a condition where a baby is born with an extra copy (either full or partial) of chromosome 21.

According to the CDC, at least 1 out of every 700 babies born in the United States is born with Down Syndrome. That is approximately 6,000 babies are born with the syndrome every year in the nation making it the most common chromosomal condition.

Children born with the condition are likely to have increased risk of heart problems and are also likely to experience retardation. The risk of chromosome abnormalities increases with the mother’s age at pregnancy.

2. Neural Tube Defects

At around the third week of pregnancy, cells on the dorsal side of the embryo start changing shape forming the neural tube. However, in some cases, the neural tube does not get completely closed leading to an opening in the spinal cord or the brain right from the early stages of development. These defects are collectively known as NTDs.

There are two types of NTDs; open NTDs (exposed brain or spinal cord at birth) which are the most common and closed NTDs (where the skin covers the spinal defect) which are much rarer. Open NTDs include spina bifida, anencephaly, and schizencephaly among others. Closed NTDs include; tethered cord and lipomeningocele. In severe cases of open NTDs, the baby may die soon after birth while in the milder cases, individuals may experience learning problems, bladder problems, and even mental retardation.

Other genetic conditions that are likely to develop in the fetus during pregnancy include;

1. Cystic fibrosis which is life-threatening and affects mostly Caucasians who have a Northern European descent.

2. Sickle Cell Disease which is common in individuals of African and Mediterranean descent.

3. Thalassemia which is also common in individuals of African and Mediterranean descent.

4. Tay-Sachs Disease which is common among those with French Canadian and Eastern European Jewish roots.

5. Fragile X syndrome which is known to cause developmental issues and even mental retardation.

Differentiating Between Genetic Screening and Genetic Testing

It is important to note that there is a distinct difference between genetic screening and genetic testing (also known as diagnostic testing).

Genetic Screening

Genetic Screening

Genetic screening tests are those that help to determine or predict the risk of a fetus contracting certain common defects and disorders. To do this, ultrasounds and blood tests are performed. They are used to predict the chances of the fetus having a chromosomal abnormality as well as a few other defects. However, these tests are merely predictive and can therefore not tell with complete certainty that the defects are present in the baby. One of the advantages of this kind of genetic testing is that they do not pose any risks to the mother or the fetus.

On the other hand, this type of genetic testing is disadvantageous in that they are not diagnostic. Instead, these tests are used to determine the need for further diagnostic genetic testing during the pregnancy. According to experts, pregnant women who get positive screening results should follow this up with counseling as well as diagnostic tests which are more reliable and more accurate.

Diagnostic Genetic Testing

Many medical experts recommend having these tests when there is a positive screening result though of course, any pregnant woman may choose to take the tests voluntarily. They are much more invasive than genetic screening tests in that they involve the insertion of a needle in the area around the pregnancy. They are used to tell with much more accuracy and certainty, the presence of a chromosomal disorder or other inherited disorders in the fetus.

The biggest advantage of this kind of genetic testing is that they can detect a greater number of birth defects and with greater accuracy and certainty. On the downside, however, these tests are much more invasive than screening tests and also pose a small risk of miscarriage.

Common Genetic Screening Tests

This type of genetic testing comprises three main types of screening tests which are all offered during pregnancy

1. Sequential Screening

This is a type of genetic testing that features the combination of information obtained from two stages of pregnancy to determine the risk of a pregnancy having chromosomal abnormalities such as Down Syndrome and also the risk of open neural tube defects.

The first stage of genetic screening takes part in the first trimester and particularly between the 10th and 13th week of pregnancy. It starts with genetic counseling which is then followed up with an ultrasound measurement of the fluid at the back of the neck of the fetus. A blood test then follows the ultrasound.

The information obtained from the ultrasound and blood test is then used to give a preliminary result indicating the risk of chromosomal abnormalities in the pregnancy. Pregnant women who get significantly positive results in this stage are often advised to go straight to diagnostic testing.

However, if the results do not indicate an uncomfortably high risk of birth defects, then the pregnant woman returns for a second blood test in the second trimester and specifically between week 16 and week 18 of the pregnancy. The information from the second-trimester blood test is the combined with information obtained from the ultrasound and blood test in the first trimester, and a final risk assessment is made. The final result indicates the risk of chromosomal abnormalities like Down syndrome and trisomy 18 as well as the risk of open neural tube defects.

2. Maternal Serum Quad Screen

This screening test is also known as the quadruple screen or the multiple markers. It also screens for Down syndrome, trisomy 18 and also neural tube defects like spina bifida. It is normally performed in the second trimester and more so between the 15th and 20th week of pregnancy, In most cases, it is performed in combination with other first-trimester screening tests (sequential screening), but it is also performed alone. However, when performed alone, it is considered less accurate than sequential screening. The screen looks for four main substances; Estriol, AFP, Inhibin-A, and hCG.

High levels of AFP may indicate the possibility of the presence of neural tube defects in the fetus while low levels of AFP, as well as abnormal levels of hCG and estriol, may indicate a possibility of the presence of chromosomal disorders such as Trisomy 21 and Trisomy 18. Elevated levels of Inhibin-A also indicate a risk of Down syndrome.

3. 20-Week Ultrasound

The name is a bit misleading because this test is performed between the 18th and the 22nd week of pregnancy. Many people are familiar with this ultrasound as it is popularly used to tell the gender of the baby. However, the primary function of this ultrasound is to analyze the risk of birth defects safely. It tests for the presence of genetic disorders as well as birth defects such as;

. Heart defects

. Cleft Lip

. Kidney problems

. Limb related abnormalities

. Brain formation problems

However, it is important not to take the results of this test as the gospel truth. Indeed, there is the possibility of false positives and false negatives in the results. Further, there is always the risk of the baby having other defects and genetic conditions not detectable in the ultrasound. Ultimately, it is advisable to pursue further genetic testing.

Types of Diagnostic Genetic Testing

Under this category of tests, there are two main tests; chorionic villus sampling (CVS) and amniocentesis

1. Chorionic Villus Sampling (CVS)

Chorionic Villus Sampling

Genetic testing of this kind takes place in the first trimester and more specifically, between the 10th and 13th week of pregnancy. This test includes taking a sample of tissue from the placenta and genetically analyzing the sample.

To collect the sample, doctors may insert a needle through the abdomen of the mother or the cervix and into the placenta. Results often take between 1-2 weeks after the sample is collected.

The test is useful in determining whether the fetus has a normal number of chromosomes. It is therefore used to diagnose for the presence of chromosomal abnormalities. However, it is also possible to diagnose other genetic conditions based on family history as well as based on results from carrier screening. There is a relatively significant risk of miscarrying after CVS (approximately between 1/250 and 1/300).

However, a CVS does not assess the risk of NTDs.

2. Amniocentesis

This type of genetic testing is usually conducted in the second trimester and more specifically between the 15th and 18th week of pregnancy. However, it is possible to conduct the test at any other time during the pregnancy. Like the CVS test, amniocentesis assesses whether the fetus has a normal number of chromosomes and is therefore useful in diagnosing chromosomal abnormalities in the fetus.

The test involves the insertion of a needle through the abdomen of the mother and into the amniotic fluid. The amniotic fluid is the liquid in which the baby floats while in the uterus. A volume approximately equal to 3 spoons is then removed (through the guidance of ultrasound).

The fluid is important because it contains cells that have fallen off naturally from the fetus at various points during its development. These are cells then undergo genetic testing with results diagnosing the presence (or lack of) chromosomal abnormalities. However, there is a slight risk of miscarriage from the procedure (approximately 1/500 to 1/1000).

Both CVS and amniocentesis are outpatient procedures with appointments taking between 2 and 3 hours.

Carrier Genetic Testing

Individuals planning to get pregnant may also undergo genetic testing before the pregnancy using a method known as carrier screening. Most procedures under this type of genetic testing are based on the ethnicity of the woman planning to get pregnant as well as the father of the child. This is because ethnicity is also a risk factor for some birth defects and other disorders.

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