Aneuploidy screening evaluates the likelihood of a baby having an incorrect number of chromosomes, a condition known as aneuploidy. This means a cell contains either too many or too few chromosomes, deviating from the typical count of 46. Screening tests do not provide a definitive diagnosis, but rather indicate an increased or decreased chance of a chromosomal difference. The purpose of these screenings is to help parents understand potential risks and make informed decisions about further testing or care.
Understanding Chromosomal Abnormalities
Chromosomes are thread-like structures found within the nucleus of every cell, carrying our genetic information in the form of DNA. Humans have 46 chromosomes arranged in 23 pairs, with one chromosome from each pair inherited from each parent. Maintaining this precise number is important for proper development and bodily function.
This condition often arises from errors during cell division before fertilization. The most common type of aneuploidy is trisomy, which means there is an extra copy of a chromosome, resulting in a total of 47 chromosomes instead of 46. While many aneuploidies are not compatible with life and often result in miscarriage, some can lead to live births.
Common trisomies include Trisomy 21, also known as Down syndrome, where an individual has an extra copy of chromosome 21. Trisomy 18, or Edwards syndrome, involves an extra copy of chromosome 18, and Trisomy 13, or Patau syndrome, results from an extra copy of chromosome 13. These trisomies are the conditions that prenatal screening tests aim to identify.
Types of Prenatal Screening Tests
Several methods are available for aneuploidy screening during pregnancy, each performed at different gestational stages. Non-Invasive Prenatal Testing (NIPT), also known as cell-free DNA (cfDNA) screening, is a blood test offered starting at 10 weeks of pregnancy. This test analyzes small fragments of fetal DNA that circulate in the mother’s bloodstream. By examining the proportions of DNA fragments from different chromosomes, NIPT can identify if there is an excess or deficiency, indicating a higher chance of conditions like Trisomy 21, 18, or 13.
Another option is the first-trimester combined screening, performed between 11 and 14 weeks of pregnancy. This screening combines two components: a nuchal translucency (NT) ultrasound and maternal blood tests. The NT ultrasound measures the fluid at the back of the baby’s neck, which can be increased in chromosomal conditions. The maternal blood tests measure levels of specific substances, such as pregnancy-associated plasma protein-A (PAPP-A) and human chorionic gonadotropin (hCG), whose levels can be altered with chromosomal abnormalities. Results from the ultrasound, blood tests, and maternal age calculate a risk estimate.
For those who do not undergo first-trimester screening or prefer an option later in pregnancy, the second-trimester maternal serum screening, commonly known as the quad screen, is available. This blood test is performed between 15 and 22 weeks of gestation. The quad screen measures the levels of four markers in the mother’s blood: alpha-fetoprotein (AFP), unconjugated estriol (uE3), human chorionic gonadotropin (hCG), and inhibin A. Abnormal levels of these markers can indicate an increased chance of Trisomy 21 or Trisomy 18, as well as neural tube defects.
Interpreting Screening Results
Screening results provide a risk assessment, not a diagnosis. Results are often presented as a ratio, such as “1 in 1,000,” which indicates that for every 1,000 pregnancies with this result, one might be affected by the condition. A “low-risk” result suggests a small chance of the baby having the screened condition. However, a low-risk result does not completely eliminate the possibility of a genetic condition.
Conversely, a “high-risk” result signifies an increased likelihood of a chromosomal abnormality. A high-risk result does not confirm the condition but indicates a higher chance, warranting further investigation. For example, for every 10 “high risk” NIPT results, between 3 and 8 pregnancies may have a trisomy. These screening tests are designed to identify pregnancies that may benefit from definitive diagnostic procedures.
The Role of Diagnostic Testing
When screening tests indicate a higher chance of an aneuploidy, diagnostic testing becomes the next step. Unlike screening tests, diagnostic tests provide a definitive answer regarding the presence of a chromosomal abnormality. These tests involve obtaining fetal cells or placental tissue from the pregnancy for genetic analysis.
One such diagnostic procedure is Chorionic Villus Sampling (CVS), which is performed earlier in pregnancy, between 11 and 14 weeks. During a CVS, a small sample of tissue, known as chorionic villi, is taken from the placenta. This is done by inserting a thin needle through the mother’s abdomen, guided by ultrasound, to collect the sample. The cells in the chorionic villi contain the same genetic information as the fetus, allowing for detailed chromosome analysis.
Another common diagnostic procedure is amniocentesis, performed later than CVS, after 15 weeks of pregnancy. For this procedure, a fine needle is inserted through the abdomen into the uterus to withdraw a small amount of amniotic fluid that surrounds the baby. This fluid contains fetal cells shed from the baby’s skin, which can then be analyzed to determine the baby’s chromosomal makeup. Both CVS and amniocentesis provide the genetic information needed to confirm or rule out an aneuploidy condition.