Abnormal ductus venosus flow velocity	The ductus venosus is a vessel in the fetus which allows oxygenated blood from the placenta to bypass the fetal liver and flow straight to the heart. In conditions such as Down’s syndrome the pressure in this vessel can be abnormally high.
Absent nasal bone	Absence of the bone that forms the bridge of the nose, which may be detected at ultrasound scan during early pregnancy.
Affected individuals	Those individuals who are affected by the disorder for which they are being screened.
Amniocentesis	Amniocentesis is an invasive procedure which involves taking a small sample of the amniotic fluid (liquor) surrounding the baby, using a needle which goes through the abdominal wall into the uterus, and is usually performed after 15 weeks' gestation.
Chorionic villus sampling (CVS)	Chorionic villus sampling involves taking a sample of the placental tissue using a needle which goes through the abdominal wall and uterus or a cannula through the cervix. It is usually performed between 10 and 13 weeks' gestation.
Combined test	First trimester test (up to 13 + 6 weeks of pregnancy) based on combining nuchal translucency measurement with free beta‐hCG, pregnancy‐associated plasma protein A (PAPP‐A) and the woman’s age.
Diagnostic accuracy	The amount of agreement between the information from the index test and the reference standard (see below).
Diagnostic test	A definitive test, performed after a positive screening test result that gives a diagnosis (i.e. yes or no).
Double test	Second trimester test (from 13 + 6 up to 24 weeks of pregnancy) based on the measurement of alpha‐fetoprotein (AFP), human chorionic gonadotrophin (hCG ß either free beta‐hCG or total hCG), together with the woman’s age.
First trimester	Pregnancy from conception up to 13 weeks and 6 days.
Iatrogenic	A disease or condition in a patient occurring as a result of treatment.
Index test	A test or group of tests being evaluated in a systematic review.
Integrated test	Measurements performed at different times of pregnancy combined into a single test result. Unless otherwise specified, 'integrated test' refers to the combination of nuchal translucency measurement and PAPP‐A in the first trimester, with the quadruple test (see below) in the second.
Mosaicism	This is a condition in which person has some cells containing a normal number of chromosomes, and some containing an abnormal number. The more abnormal cells there are, the greater the effect.
Multiple of the median (MOM)	The serum test concentration for a pregnant woman divided by the average (median) for unaffected pregnancies in a defined population at the same stage of pregnancy.
Quadruple test	Second trimester test (from 13 + 6 up to 24 weeks of pregnancy) based on the measurement of AFP, uE3, free beta‐hCG (or total hCG), and inhibin‐A together with the woman’s age.
Reference Standard	The best available method for establishing the presence or absence of the target disease or condition.
Second trimester	Pregnancy from 14 weeks to 28 weeks' gestation. Note that for the purposes of this Cochrane review, second trimester testing refers to the period of 14 to 24 weeks' gestation.
Tricuspid regurgitation	Leakiness of or backflow of blood through the tricuspid valve of the heart. The tricuspid valve separates the upper and lower chambers of the right side of the heart.
Triple test	Second trimester test (from 14 up to 24 weeks of pregnancy) based on the measurement of AFP, unconjugated oestriol (uE3), and hCG (either total hCG or free beta‐hCG) together with the woman's age.
Trisomy	The presence of an extra chromosome resulting in three copies of a particular chromosome instead of the normal two.
Translocation	Part of one chromosome is broken off and attached to another chromosome. This does not usually cause the individual any problems as they have a normal amount of chromosomes, but in an abnormal arrangement.  It can be passed on as an extra chromosome to offspring, resulting in conditions such as Down's syndrome.