Prenatal Genetic Testing: When to Do It and Why It’s Important
Prenatal genetic tests are a fundamental tool for monitoring fetal health during pregnancy. Thanks to their high reliability—reaching up to 99% accuracy for the most common trisomies in the case of fetal DNA testing—these exams provide valuable information about potential genetic risks.
During pregnancy, prenatal genetic tests and screening tests can be performed at different stages. Fetal DNA testing (NIPT) can be carried out as early as the 10th week of gestation, while the combined test is performed between 11+3 and 13+6 weeks, with an overall reliability of about 90%. These tests are particularly recommended when an intermediate risk emerges during first-trimester screening.
In this guide, we examine the different types of prenatal genetic tests available, when to perform them, and why they are important. We also discuss the NIPT, which analyzes fragments of the baby’s DNA released from the placenta into the mother’s bloodstream, offering results with a false-positive rate below 0.1%. We will explore which chromosomal abnormalities can be detected, including trisomy 21 (Down syndrome), trisomy 18 (Edwards syndrome), and trisomy 13 (Patau syndrome).
What Is Prenatal Genetic Testing and What Is It Used For?
Modern prenatal diagnostics offer several tools to monitor fetal development. Prenatal genetic tests are examinations designed to identify possible genetic abnormalities in the unborn baby before birth.
Difference Between Genetic Tests and Prenatal Screening Tests
It is essential to understand the distinction between diagnostic tests and screening tests.
Prenatal screening tests are non-invasive examinations that estimate the probability that the fetus has certain chromosomal abnormalities, without providing a definitive diagnosis. These tests identify risk based on statistical models; they are safe but express results in terms of probability.
In contrast, diagnostic genetic tests are usually invasive and carry a small risk for the fetus. However, they provide a definitive diagnosis of genetic abnormalities. These tests can detect all trisomies, many other chromosomal abnormalities, and hundreds of Mendelian genetic disorders.
The most common screening tests include:
- Fetal DNA test (NIPT – Non-Invasive Prenatal Test)
- Combined test (nuchal translucency + serum markers)
- Maternal serum screening
Fetal DNA testing can be performed through a simple maternal blood draw starting from the 10th week of pregnancy, while serum marker screening is typically carried out between 15 and 20 weeks of gestation.
What Can Be Detected: Chromosomal and Genetic Abnormalities
Prenatal genetic tests can identify a wide range of chromosomal and genetic abnormalities. Aneuploidies, which are alterations in the number of chromosomes, are the most commonly investigated conditions.
Screening tests primarily focus on:
- Trisomy 21 (Down syndrome)
- Trisomy 18 (Edwards syndrome)
- Trisomy 13 (Patau syndrome)
Together, these account for approximately 70% of known chromosomal abnormalities. The likelihood of having a baby with Down syndrome increases with maternal age.
More advanced tests may also detect:
- Sex chromosome abnormalities (e.g., Turner syndrome, Klinefelter syndrome)
- Microdeletions (loss of small DNA segments)
- Rare trisomies, such as those involving chromosomes 9 and 16
Fetal DNA testing has demonstrated over 99% accuracy for trisomies 21 and 13, offering a higher likelihood of correctly identifying affected fetuses and a lower chance of false positives.
Why Prenatal Genetic Testing Is Important
Prenatal genetic testing is important because approximately 3–5% of pregnancies are affected by congenital defects or genetic diseases. These tests allow for early identification of potential issues, enabling parents to make more informed decisions.
Screening tests such as the combined test and NIPT help identify pregnancies at higher risk for chromosomal abnormalities more accurately, reducing unnecessary use of invasive diagnostic procedures. This is especially important given that invasive tests carry a miscarriage risk of approximately 0.5–1%.
Despite their high accuracy, it is crucial to emphasize that fetal DNA testing is not diagnostic. A positive result must always be confirmed through invasive diagnostic procedures such as amniocentesis or chorionic villus sampling (CVS).
In summary, prenatal genetic testing offers couples the opportunity to assess fetal health early. In the absence of specific risk factors, screening tests that include NIPT are often recommended, as they significantly reduce unnecessary invasive procedures and associated risks.
When Is Prenatal Genetic Testing Recommended?
The decision to undergo prenatal genetic testing depends on several risk factors that may increase the likelihood of chromosomal or genetic abnormalities.
Advanced Maternal Age
Maternal age is one of the most significant risk factors. While chromosomal abnormalities can occur at any age, their frequency increases significantly after age 35.
Statistical data show:
- At 35 years, the risk of Down syndrome is 1 in 385
- At 40 years, the risk increases to 1 in 106
- At 45 years, it rises to 1 in 30
For this reason, prenatal diagnosis is commonly offered to women who will be 35 or older at delivery. However, age alone is considered an arbitrary criterion, and testing may also be recommended for younger women with additional risk factors.
NIPT is suitable for women both under and over 35, provided there is no known genetic disease in the family and no fetal structural anomalies detected on ultrasound.
Family History of Genetic Disorders
Prenatal genetic testing is strongly recommended in cases such as:
- A previous child with a chromosomal abnormality
- One partner carrying a balanced chromosomal rearrangement
- Presence of supernumerary chromosomal markers
- Chromosomal mosaicism or sex chromosome aneuploidy in a parent
- Family history of genetic disorders or neural tube defects
In these situations, parents often opt directly for invasive diagnostic tests rather than non-invasive screening.
Suspicious Ultrasound Findings
Prenatal genetic testing is also recommended when ultrasound examinations reveal abnormalities, such as:
- Increased nuchal translucency (≥3 mm)
- Structural fetal anomalies
- Positive first-trimester screening (Bi-test)
When fetal malformations are suspected, invasive diagnostic testing is generally required, as NIPT alone is not sufficient.
Pregnancy Through Assisted Reproduction (ART)
Pregnancies achieved through assisted reproductive technologies (ART) may warrant prenatal genetic testing for several reasons:
- Genetic conditions can be identified before embryo transfer through preimplantation genetic testing (PGT)
- PGT is particularly useful in cases of advanced maternal age
- Other indications include repeated ART failures, recurrent miscarriages, or severe male infertility
Scientific consensus generally recommends PGT for women over 38 years, while its role in women aged 35–38 remains less clearly defined.
Main Prenatal Genetic Tests Available
Fetal DNA Test (NIPT)
- Non-invasive
- Performed from the 10th week
- Accuracy over 99% for trisomy 21 and 18
- False-positive rate below 0.1%
- Screens for trisomies 13, 18, 21 and sex chromosome abnormalities
➡️ Not diagnostic; positive results require confirmation
Combined Test
- Performed between 11+3 and 13+6 weeks
- Combines ultrasound and blood markers
- Approximately 90% detection rate
- False-positive rate around 5%
Amniocentesis
- Performed between 15–18 weeks
- Diagnostic test
- Miscarriage risk below 1%
Chorionic Villus Sampling (CVS)
- Performed between 10–13 weeks
- Diagnostic test
- Miscarriage risk 1–2%
Preimplantation Genetic Testing (PGT)
- Performed in ART cycles
- Detects chromosomal abnormalities and monogenic disorders before implantation
Focus on the Fetal DNA Test (NIPT)
The NIPT analyzes fragments of fetal DNA circulating in maternal blood. Advanced sequencing technologies and bioinformatic algorithms compare chromosomal fragments to detect abnormalities.
What It Detects
- Trisomy 21, 18, 13
- Sex chromosome aneuploidies
- Some microdeletions (e.g., DiGeorge syndrome)
Reliability and Limitations
- Sensitivity >99% for trisomy 21
- Very low false-positive rate
- Screening test only → confirmation required
What to Do After the Results
False Positives and False Negatives
No test is 100% accurate. Screening tests may occasionally yield false results or be inconclusive due to low fetal DNA fraction.
When Diagnostic Testing Is Needed
Any positive or borderline screening result must be confirmed with invasive testing before making clinical or legal decisions.
Importance of Genetic Counseling
Genetic counseling is essential both before and after testing to explain results, limitations, and next steps.
Psychological Considerations
Receiving a positive result can be emotionally distressing. Psychological support is strongly recommended to help parents cope and make informed decisions.
Conclusion
Prenatal genetic testing plays a crucial role in modern pregnancy care. Understanding available options allows expectant parents to make informed, confident decisions.
The NIPT, in particular, stands out for its high accuracy, safety, and early availability. However, it remains a screening tool and must be followed by diagnostic testing when results are positive.
Ultimately, prenatal genetic tests provide reassurance in most cases and valuable preparation in others. Pre- and post-test genetic counseling remains essential to ensure results are correctly interpreted and decisions are made with clarity and support.
