Amniocentesis is a prenatal diagnostic procedure that involves obtaining a small sample of amniotic fluid from the uterus for testing. This fluid contains fetal cells with genetic information, allowing healthcare providers to assess the fetus’s genetic makeup. Chromosomal microarray is a technology that offers a detailed look at the fetal genome. This article explains what amniocentesis microarray tests for, including its capabilities and limitations.
Understanding Amniocentesis Microarray
Amniocentesis involves a healthcare provider inserting a thin needle through the pregnant person’s abdomen and uterus to collect amniotic fluid. This procedure is typically performed between 15 and 20 weeks of pregnancy. The fetal cells from this fluid are then used for genetic analysis, including chromosomal microarray.
Chromosomal microarray, also known as chromosomal microarray analysis (CMA), is a laboratory technique that examines the entire genome for missing or extra pieces of genetic material. This technology uses a microchip platform with thousands of DNA probes. By comparing the fetal DNA to a reference sample, the microarray can detect very small deletions or duplications, known as copy number variants (CNVs). This high-resolution approach identifies subtle chromosomal imbalances not visible with traditional methods.
Genetic Conditions Detected
Amniocentesis microarray detects microdeletions and microduplications. These are tiny segments of chromosomes that are either missing or present in extra copies. Such changes, even if small, can disrupt gene function and lead to developmental and health issues.
Many syndromes are caused by these submicroscopic changes. For instance, DiGeorge syndrome results from a microdeletion on chromosome 22, leading to heart problems, developmental delays, and immune system deficiencies. Williams syndrome is associated with a microdeletion on chromosome 7, characterized by distinctive facial features, intellectual disability, and cardiovascular issues.
Prader-Willi syndrome and Angelman syndrome are linked to changes in chromosome 15. The specific syndrome depends on whether the genetic material is missing or altered on the paternally or maternally inherited chromosome. Cri-du-chat syndrome arises from a partial deletion on chromosome 5, known for a characteristic high-pitched cry, intellectual disability, and developmental delays. Microarray can also detect larger chromosomal abnormalities, such as Down syndrome (Trisomy 21), Trisomy 13, and Trisomy 18. Its main benefit is identifying smaller, submicroscopic changes.
Microarray Compared to Other Genetic Tests
Amniocentesis microarray offers higher resolution compared to traditional karyotyping. Karyotyping involves visually examining chromosomes under a microscope to identify large numerical changes or structural rearrangements. This method relies on banding patterns to detect abnormalities.
Microarray operates at a much finer scale, capable of detecting genetic changes as small as 500 kilobases, which are several orders of magnitude smaller than what can be seen with standard karyotyping. This enhanced resolution allows microarray to identify microdeletions and microduplications that microscopic examination would miss. As a result, microarray is often a preferred first-tier test in prenatal diagnosis due to its increased ability to identify clinically significant genetic variations. Karyotyping may still be useful for detecting balanced rearrangements.
What Amniocentesis Microarray Does Not Detect
Despite its advanced capabilities, amniocentesis microarray has specific limitations. The test does not detect single-gene disorders, which are caused by mutations within a single gene. Conditions such as cystic fibrosis, sickle cell anemia, or Tay-Sachs disease require different molecular tests if suspected.
Microarray also does not identify neural tube defects, such as spina bifida or anencephaly. These structural birth defects are usually screened for through ultrasound examinations and maternal serum alpha-fetoprotein (AFP) levels. Additionally, the test may not reliably detect low-level mosaicism, a condition where a genetic abnormality is present in only a small percentage of the cells.
Balanced chromosomal rearrangements, where genetic material is rearranged but no amount is gained or lost, are also not typically detected by microarray. Finally, microarray results can sometimes reveal variants of uncertain clinical significance (VUS), which are genetic changes whose impact on health is not yet fully understood. These VUS findings can lead to uncertainty for families.