A fetal chromosomal aneuploidy test is a medical assessment performed during pregnancy to identify if a developing baby has an abnormal number of chromosomes. This test provides information about the fetus’s genetic makeup, helping expectant parents and healthcare providers prepare for potential health considerations.
Understanding Chromosomal Aneuploidy
Chromosomes are thread-like structures within every cell’s nucleus, carrying genetic information. Humans typically have 46 chromosomes arranged in 23 pairs, with 23 from each parent. These chromosomes contain genes that dictate an individual’s development and bodily functions.
Aneuploidy is a condition with an abnormal number of chromosomes, meaning an extra or missing copy. This alteration can occur during egg or sperm formation or early embryonic cell division. An imbalance in chromosome number can disrupt normal cellular processes and lead to developmental challenges. While many aneuploidies result in early pregnancy loss, some can lead to live births with varying physical and intellectual differences.
Types of Fetal Aneuploidy Tests
Fetal aneuploidy testing involves different methods, broadly categorized as screening or diagnostic tests. Each type provides information about the fetus’s chromosomal status.
Non-Invasive Prenatal Testing (NIPT)
Non-Invasive Prenatal Testing (NIPT) is a screening test that analyzes cell-free DNA (cfDNA) fragments in the pregnant person’s blood. These cfDNA fragments primarily originate from the placenta and are usually identical to fetal DNA. Performed at or after 10 weeks of gestation, the test requires enough fetal DNA in the maternal bloodstream for accurate analysis.
A blood sample is drawn and sent to a laboratory, which counts cfDNA fragments from each chromosome. An abnormal amount of DNA fragments from a particular chromosome can indicate an increased likelihood of aneuploidy. NIPT is non-invasive, carrying no risk of miscarriage to the fetus, unlike diagnostic procedures.
Chorionic Villus Sampling (CVS)
Chorionic Villus Sampling (CVS) is an invasive diagnostic test performed between 10 and 13 weeks of pregnancy. It involves taking a small tissue sample from the placenta. The cells from the chorionic villi, tiny finger-like projections on the placenta, share the same genetic makeup as the fetus.
CVS can be performed transcervically, by inserting a thin tube or forceps through the vagina and cervix, or transabdominally, by inserting a needle through the abdomen into the uterus. An ultrasound guides both procedures for precise placement.
Amniocentesis
Amniocentesis is an invasive diagnostic test involving the collection of amniotic fluid. This fluid surrounds the fetus and contains fetal cells with the baby’s genetic information. The test is typically performed between 15 and 20 weeks of pregnancy.
During the procedure, a needle is inserted through the pregnant person’s abdomen into the amniotic sac, guided by ultrasound. A small amount of fluid is withdrawn and sent to a laboratory for analysis, where cells are grown and examined for chromosomal disorders.
Conditions Detected and Interpreting Test Results
Fetal aneuploidy tests screen for or diagnose common chromosomal conditions, guiding pregnancy management. Identified aneuploidies include trisomies and sex chromosome aneuploidies.
Trisomy occurs when there is an extra copy of a chromosome (three copies instead of two). Common autosomal trisomies compatible with live birth are Trisomy 21 (Down syndrome), Trisomy 18 (Edwards syndrome), and Trisomy 13 (Patau syndrome).
Down syndrome, affecting about 1 in 700 births, is characterized by intellectual disabilities and potential physical malformations. Trisomy 18 and Trisomy 13 are associated with severe intellectual disabilities and birth defects, often leading to a short lifespan.
Sex chromosome aneuploidies involve an abnormal number of X or Y chromosomes. Examples include Monosomy X (Turner syndrome), where a female has only one X chromosome, affecting approximately 1 in 2,500 females and potentially causing short stature and infertility.
Klinefelter syndrome (47, XXY) affects males (about 1 in 500 to 1,000 births) and can lead to increased height and fertility issues. Triple X syndrome (47, XXX) in females and Jakob syndrome (47, XYY) in males are other sex chromosome aneuploidies, often with mild or no symptoms.
Test results are typically reported as “low risk,” “high risk,” “positive,” or “negative.” For screening tests like NIPT, a “low risk” or “negative” result suggests a decreased likelihood of the condition but does not guarantee a healthy baby. A “high risk” or “positive” NIPT result indicates an increased chance of aneuploidy but is not a definitive diagnosis.
For instance, a high probability NIPT result for Trisomy 21 may have a positive predictive value of approximately 90%, meaning there is still a 10% chance the result is incorrect. Diagnostic tests like CVS and amniocentesis provide a definitive “positive” or “negative” answer regarding a chromosomal condition’s presence.
Important Considerations and Next Steps
Understanding the accuracy and limitations of fetal aneuploidy tests is important for informed decision-making. Screening tests, such as NIPT, are highly accurate for detecting common trisomies, with detection rates for Trisomy 21, 18, and 13 often exceeding 98-99% and low false-positive rates (typically around 0.13%).
However, NIPT is a screening tool, not a diagnostic test, meaning false positives and false negatives can occur (e.g., a false negative rate of about 0.09% for Trisomy 21). If a screening test like NIPT indicates a “high risk” result, diagnostic testing (CVS or amniocentesis) is often recommended to confirm findings.
This is because NIPT analyzes placental DNA, which may not always perfectly reflect the fetal karyotype due to chromosomal mosaicism. A detailed ultrasound may also be recommended to look for structural anomalies.
Genetic counseling is a valuable resource before and after testing. A genetic counselor can explain test implications, discuss testing options, and help individuals understand their choices. They provide support and coordinate additional testing or follow-up care, helping families navigate information and make pregnancy decisions.