What is Prenatal Genetic Testing?

Understanding the different types of prenatal genetic testing. 

Several prenatal genetic tests can help parents understand more about their child’s genetic health before they are born.

Carrier screening

Parents also have the option of carrier testing, either before or during a pregnancy, which will reveal whether the parents are themselves carriers of a specific gene or DNA mutation that might cause their child to be born with a specific genetic syndrome or not (depending on the particular gene mutation, and the mode of inheritance of the syndrome involved). Carrier genetic testing can identify syndromes such as Fragile X, Tay Sachs, and Cystic Fibrosis. 

Prenatal genetic testing

Prenatal genetic testing may involve many different methods from an ultrasound to identify physical signs of a rare syndrome, testing blood samples from the pregnant mother, and also testing samples from the amniotic fluid surrounding the developing fetus.

These tests may occur in the first, second, or third trimester of the pregnancy. The tests may identify a wide range of possible genetic syndromes or rare diseases. Some of these tests are standard, and as part of the scheduled antenatal care of pregnant women, some of them are optional. 

First-trimester prenatal genetic testing usually takes place between the 10th and 13th weeks of the pregnancy. This sequential testing includes a blood sample taken from the pregnant mother and an ultrasound which measures the thickness of the space at the back of the neck of the developing fetus. This ultrasound is known as a nuchal translucency test. The blood test results, combined with the measurements from the ultrasound, show the risk of the fetus being born with Down syndrome. This genetic testing is a part of routine antenatal care in most developed nations and is offered to all pregnant women regardless of age, or family history. 

Prenatal genetic testing in the second trimester may take two forms:

• The quad blood test involves taking a blood sample from the pregnant mother. It is usually done during the 15th and 22nd weeks of pregnancy. This sample is then screened for Down syndrome, Edwards syndrome, and other possible nuchal translucency disorders. 
• A fetal anomaly ultrasound is usually performed between weeks 18 and 24 of the pregnancy. This specialist ultrasound checks for possible defects in the major organs and systems of the body. The ultrasound will also check the developing fetus’s facial features for any anomalies that might indicate a rare genetic syndrome or disease. 

Combining the results of both the first and second trimester screenings allows medical professionals and parents to understand more about their developing baby’s health and if there are any potential future health conditions to address. 

Amniocentesis is a form of prenatal genetic testing recommended for mothers over a certain maternal age, with a family history of genetic syndromes or rare diseases, or if earlier screening indicates a possible problem that requires more advanced testing. The testing involves taking a sample of amniotic fluid surrounding the developing fetus, which is then screened. This fluid contains fetal tissues and cells. There is a degree of risk involved with amniocentesis. The procedure can increase the risk of miscarriage and has been found to do so in 1 out of every 100 tested. To lower the risk, amniocentesis is usually not carried out before the 15th week of pregnancy. 

CVS is a type of genetic testing often offered as an alternative to amniocentesis. It can be carried out earlier than amniocentesis, giving parents more time to consider all of their options when facing a possible diagnosis. It is usually carried out between weeks 11 and 14 of pregnancy. The risk of miscarriage is the same, with 1 out of every 100 women suffering a loss due to the procedure. 

Non-invasive genetic testing

Parents may now also opt for prenatal genetic testing known as NIPT, or non-invasive prenatal genetic testing. A blood sample from the mother is screened for a wide variety of genetic syndromes and chromosomal diseases and can be performed as early as 9 weeks, but no later than 13 weeks and 6 days in the pregnancy. NIPT has a degree of accuracy of 99%, making it an increasingly popular choice of testing for parents to be. 

Despite the improving accuracy of prenatal genetic testing, specifically with the development of NIPT testing, the testing is not necessarily able to provide a confirmed diagnosis for a developing fetus. The testing may indicate a genetic syndrome or possible medical condition, but genetic testing is also required after birth for a confirmed diagnosis. Prenatal genetic testing cannot confirm the exact symptoms and features a child may be born with, nor is it able to predict the severity of these symptoms.

Genetic Counseling

As with all genetic testing, the role of genetic counseling should not be underestimated when it comes to prenatal genetic testing. Ensuring potential parents receive the information and support they need if and when they face the possibility of a child with a rare genetic syndrome or disease is crucial.