Glossary of Terms
Amniocentesis
Amniocentesis is a diagnostic test that allows a doctor to take a sample from your pregnancy to look at the chromosomes in detail. In this way, amniocentesis gives a more definitive answer for most chromosome conditions. This includes the conditions we have already discussed, and many other, rarer chromosome conditions. Amniocentesis can be performed in the second trimester of pregnancy or later. During amniocentesis, a small needle is inserted through a pregnant woman's abdomen, and a small amount of the fluid from the amniotic sac is removed. The sample is sent to a laboratory where the chromosomes are analyzed. Testing can also be performed on amniotic fluid to detect a condition that involves an opening in the developing baby's spine or belly. Additional testing for other conditions can be performed on the sample, if indicated based on your specific family or medical history. There is a risk of complications, including a small risk for miscarriage, from having amniocentesis.
Autism
A condition that causes a person difficulty with communicating and forming relationships with the world around them.
Cell-free DNA (cfDNA)
Small fragments of DNA from the pregnancy, mostly the placenta, that circulate in the pregnant person’s blood.
Chorionic villus sampling (CVS)
CVS is a diagnostic test that allows a doctor to take a sample from your pregnancy to analyze the chromosomes in detail. In this way, CVS provide a more definitive answer for most chromosome conditions. This includes the conditions we have already discussed, and many other, rarer chromosome conditions. CVS can be performed by a specialist between weeks 10-14 of pregnancy. In this procedure, a small sample of the placenta is removed either by inserting a small needle through the pregnant woman's abdomen or by inserting a tube (called a catheter) through the cervix. The sample is sent to a laboratory where the chromosomes are analyzed. Additional testing for other conditions can be performed on the sample from the CVS, if indicated based on your specific family or medical history. There is a risk of complications, including a small risk for miscarriage, from having CVS.
Chromosomal conditions
Conditions, like Down syndrome, caused by an extra or missing copy of a chromosome. Chromosomes carry our genetic information.
Congenital malformation
Physical differences; a problem with the growth or development of a baby that happens during pregnancy.
Copy Number Variants (CNVs)
A genetic difference in which a section of DNA is missing or has extra copies. CNVs are common, and many do not affect health, although some may be linked to a genetic condition.
Detection rate
The percent of time a test finds a condition when it is present.
Down syndrome
People with Down syndrome experience learning difficulties, usually mild to moderate. Most people with Down syndrome can communicate verbally and take care of their basic needs (such as bathing, toileting, dressing, etc.). The ages when they reach certain milestones vary among different people and are impossible to predict. People with Down syndrome have physical characteristics, such as an upward slant to the eyes. Each person with Down syndrome will have these characteristics to different degrees and will share many characteristics of their family members. People with Down syndrome have a higher risk for certain medical conditions, such as heart defects, hearing problems, thyroid problems, and childhood leukaemia. Most children with Down syndrome can attend mainstream school in their early years. In adulthood, many people with Down's syndrome can hold jobs, live independently with support and enjoy a good quality of life.
Edwards syndrome
People with Edwards syndrome have severe learning difficulties and multiple physical differences. Some of the common features of Edwards syndrome include heart defects, brain abnormalities, cleft lip and palate, and low birth weight. Edwards syndrome can be life-threatening in the early months and years of life, however, some people with this condition can live years or even decades.
False positive result
A woman receives a positive screening result but the pregnancy is NOT affected with the condition being screened for.
Fetal Fraction
Percentage of cfDNA from the pregnancy compared to the total amount of cfDNA in the mother’s blood sample.
First trimester screening (FTS)
First trimester screening [FTS] (also called the first trimester combined test) is a screening test that is performed in the first trimester of pregnancy and involves a combination of ultrasound and a blood test. FTS estimates the chance of Down's syndrome and trisomy 18, and sometimes trisomy 13, in a particular pregnancy but does not test for all possible chromosome conditions. FTS is safe to both the mother and the pregnancy and does not carry a risk for miscarriage. As a screening test, FTS does not provide a definitive answer. Follow-up testing, such as NIPT, CVS, or amniocentesis, should be considered if the FTS results indicate an increased chance of a chromosome condition.
Learning difficulty
Ability to learn and develop is slower than expected.
Mosaicism
When some cells have different genetic information from other cells in the same pregnancy or person. The effects of the change depend on the genetic change, the cells affected and the approximate amount of cells that have the change.
Negative screening result
A reassuring result which means the chance the condition is present is low. May be reported as "negative" or "low risk" or "not detected".
Patau syndrome
People with Patau syndrome have severe learning difficulties and multiple physical differences. Some of the common features of Patau syndrome include heart defects, abdominal defects, brain abnormalities, cleft lip and palate, and other features. Patau syndrome can be life-threatening in the early months and years of life, however, some people with this condition can live years or even decades.
Positive screening result
The chance the condition is present is higher. More testing would be needed to confirm whether the condition is present. May be reported as "positive" or "high risk" or "aneuploidy detected"
Rare Autosomal Trisomies (RAT)
An extra copy of a chromosome other than chromosomes 13, 18, 21, X or Y.
RhD or Rhesus Factor D
A protein that may be found on the surface of red blood cells. If you have this protein, your blood type is RhD positive (such as A+); if you do not, your blood type is RhD negative (such as A−). RhD status is inherited and is important for blood transfusions and pregnancy.
Second trimester screening
Second trimester screening (including maternal serum quadruple screening [quad screen]) is a screening test that is performed in the second trimester of pregnancy and involves a blood test from the pregnant woman. Second trimester screening estimates the chance of Down syndrome and trisomy 18 in a particular pregnancy but does not test for all possible chromosome conditions. In addition, second trimester screening gives information regarding the development of the spine or certain rare genetic conditions in the pregnancy. Second trimester screening is safe to both the mother and the pregnancy and does not carry a risk for miscarriage. As a screening test, second trimester screening does not provide a definitive answer. Follow-up testing, such as NIPT, CVS, or amniocentesis, may be offered if the second trimester screening results indicate an increased chance of a chromosome condition.
Single-gene condition
A genetic condition caused by a change in a single gene. This change may be inherited from a parent or occur for the first time in an individual.
Ultrasound
Ultrasound is a screening test that may be performed at various times throughout pregnancy. A first trimester ultrasound may be performed between 10-14 weeks of pregnancy to measure the nuchal translucency (NT) or thickness of the back of the baby's neck. A detailed ultrasound can also be performed at around 20 weeks of pregnancy to look at the baby's growth and development. Although ultrasound may identify some features of common chromosomal conditions, a normal ultrasound does not eliminate the chance of a chromosomal condition. If a chromosomal condition is suspected based on ultrasound, follow-up testing, such as NIPT, CVS, or amniocentesis, may be offered.
Additional Resources
Every Stage Health
From the American College of Obstetrics and Gynecologists (ACOG) Library of patient-friendly resources from ACOG