Aneuploidy screening during pregnancy refers to a set of tests offered to expectant parents to assess the likelihood of the fetus having certain chromosomal conditions. These screenings are a routine part of prenatal care, available to all pregnant individuals regardless of age. Their primary purpose is to identify pregnancies that may have an increased risk, allowing for informed decisions and further evaluation if desired. It is important to understand that these tests are designed to provide a risk assessment, not a definitive diagnosis of a chromosomal disorder.
Understanding Aneuploidy
Aneuploidy describes a condition where an individual has an abnormal number of chromosomes, meaning there are either too many or too few. Human cells typically contain 46 chromosomes, arranged in 23 pairs, with one chromosome from each pair inherited from each parent. When aneuploidy occurs, this precise balance is disrupted, leading to various genetic conditions.
The most commonly screened aneuploidies in pregnancy include Trisomy 21, Trisomy 18, and Trisomy 13. Trisomy 21, widely known as Down syndrome, results from an extra copy of chromosome 21. Trisomy 18, or Edwards syndrome, involves an extra chromosome 18, while Trisomy 13, or Patau syndrome, is caused by an extra chromosome 13. These extra chromosomes can lead to developmental and physical differences that vary in severity depending on the specific chromosome involved.
Types of Aneuploidy Screening
Several methods are available for aneuploidy screening, each performed at different stages of pregnancy and involving distinct procedures. These screening tests are voluntary, providing expectant parents with information to help them make decisions about their pregnancy.
Non-Invasive Prenatal Testing (NIPT), also known as cell-free DNA testing, analyzes fragments of placental DNA circulating in the mother’s bloodstream. This test can be performed early in pregnancy, typically at or after 10 weeks of gestation. NIPT offers high accuracy for detecting common aneuploidies like Trisomy 21, 18, and 13, and can also determine fetal sex and sex chromosome aneuploidy. Its advantage lies in its non-invasive nature and superior performance compared to traditional serum screenings, often resulting in fewer false positives.
First Trimester Screening combines two components, usually performed between 9 and 13 weeks of gestation. It includes a blood test that measures specific biochemical markers, namely pregnancy-associated plasma protein A (PAPP-A) and human chorionic gonadotropin (hCG). In conjunction with the blood test, an ultrasound measurement of the nuchal translucency (NT) is performed, which assesses the fluid-filled space at the back of the fetal neck. An increased NT measurement can indicate an elevated risk for Down syndrome and other aneuploidies, with this combined approach detecting approximately 82% to 87% of Trisomy 21 cases.
Second Trimester Screening, often referred to as the Quad Screen, is a blood test typically conducted between 15 and 22 weeks of gestation. This test measures the levels of four specific substances in the mother’s blood: alpha-fetoprotein (AFP), unconjugated estriol, human chorionic gonadotropin (hCG), and inhibin A. The results are used to calculate a risk assessment for Trisomy 21 and Trisomy 18, as well as open neural tube defects. The Quad Screen can detect about 81% of Trisomy 21 cases.
Interpreting Screening Results and Next Steps
A “positive” or “high-risk” aneuploidy screen result indicates an increased likelihood that the fetus may have a chromosomal condition, but it does not confirm a diagnosis. Conversely, a “negative” or “low-risk” result suggests a reduced chance of such a condition, though it does not guarantee a completely typical number of chromosomes.
If a screening test indicates an increased risk, genetic counseling is typically recommended. A genetic counselor can help parents understand the specific implications of the results, discuss the likelihood of a chromosomal condition, and explain available options. This counseling is an important step in navigating the next phase of prenatal care.
To obtain a definitive diagnosis, invasive diagnostic tests are offered. These include chorionic villus sampling (CVS) and amniocentesis. CVS is usually performed between 10 and 13 weeks of gestation, involving the collection of a small piece of placental tissue. Amniocentesis is typically performed starting at 15 weeks of gestation, where a small amount of amniotic fluid surrounding the fetus is extracted. Both procedures carry a small risk, including a low chance of miscarriage.