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Glossary

Alpha Feto Protein:

Product of the fetal yolk sac and fetal liver. AFP is a screening tool for neuro tube defects. Most effectively measured between 15 – 20 weeks gestation. Higher AFP levels are found in pregnancies affected with neuro tube defects. Pregnancies affected with Down’s syndrome usually have lower levels than normal.

Amniocentesis:

An invasive procedure undertaken from about 15 completed weeks onwards to obtain a sample of amniotic fluid surrounding the fetus. A needle is passed through the mother’s abdomen into the uterus, under continuous ultrasound guidance, and a sample of fluid is withdrawn. The fluid and cells within it can be tested for certain conditions such as Down’s syndrome and other chromosomal and inherited disorders. Out of 100 women who have this test from 15 weeks it is likely that one will miscarry as a direct consequence of the procedure.

Amnionicity:

Determining in twins, triplets the number of amniotic sacs.

Anencephaly:

A defect in the closure of the neural tube during fetal development. The neural tube is a narrow channel that folds and closes between the 3rd and 4th weeks of pregnancy to form the brain and spinal cord of the embryo. Anencephaly occurs when the "cephalic" or head end of the neural tube fails to close, resulting in the absence of a major portion of the brain, skull, and scalp. Infants with this disorder are born without a forebrain and a cerebrum and do not survive for long after birth.

Anhydramnios:

Absence of amniotic fluid.

Anomaly:

An aberration or change often used related to a gene that may or may not result in a disease or condition.

Blastocyst:

The blastocyst is characterized by a group of cells, called the inner cell mass (also called embryoblast) and the mentioned trophoblast (the outer cells). The inner cell mass gives rise to the embryo proper, the amnion, yolk sac and allantois, while the trophoblast will eventually form the placenta. The blastocyst can be thought of as a ball of a (mostly single) layer of trophoblast cells, with the inner cell mass attached to this ball's inner wall.

Chorionicity:

The number of separate functioning placentas in a twin pregnancy. Non identical twins are always dichorionic. Even if their placentas join their circulation will be separate.

Combined Test:

Between 11 and 13 +6 weeks of pregnancy, a combination of the nuchal scan measurement and a blood sample from the mother which measures the concentration of pregnancy associated plasma protein A (PAPP A) and free beta human chorionic gonadotrophin (free beta hcg). Together with the mother’s age and the gestation of the pregnancy, these are used to estimate the chances that the fetus is affected.

Congenital Anomaly:

An anomaly present at birth, although not necessarily hereditary.

Congenital Diaphragmatic Hernia:

The diaphragm is a thin dome-shaped muscle which separates the chest cavity (lungs and heart) from the abdominal cavity (intestines, stomach, liver, etc.). The diaphragm normally forms between the 7th and 10th week of pregnancy.

In babies with a Congenital Diaphragmatic Hernia (CDH), the diaphragm does not form completely. There is a hole in the diaphragm that allows part of the organs that are normally found in the abdomen to move up into the chest cavity. This hole is found on the left side of the body about 85% of the time. It may also occur on the right side or in the middle. A CDH can often be identified during a prenatal ultrasound, but some cases of CDH are not found until later in pregnancy or after birth.

Crown Rump Length:

Linear measurement taken from the crown of the head to the rump.

Diagnostic Test:

Refers to the analytical process involved in obtaining a result. For example the diagnostic test on an amniocentesis sample is the karotype.

Diamniotic:

When twins have their own separate amniotic sac surrounding each of them.

Dichorionic:

Twins that have two separate placentas.

Dizygotic:

A twinning term, having two amniotic sacs which can occur in both early and late monozygotic twinning. In a monozygotic twinning event (one fertilised egg and a single spermatozoa, form a single zygote) which occurs early (within 2 days of fertilization) where each embryo then develops separately its own chorionic sac (placenta) and amnionic sacs (dichorionic diamniotic). Later splitting (more than 2 days after fertilization) may result in a shared placenta and only duplication of the embryonic amniotic sacs (monochorionic diamniotic twins).

Downs Syndrome: Same as trisomy 21

Trisomy 21: Down’s Syndrome: A disorder caused by the presence of an extra copy (three instead of two) of chromosome 21. It affects all population groups and is distinguished by a number of features occurring together including low muscle tone, a face that appears flatter with eyes slanting upward, small ears and an unusually wide neck and deep crease across the palm of the hand. Some may have heart problems or visual problems or may develop Alzheimer’s disease. Although people with Down’s syndrome have learning difficulties, these vary in severity.

Duodenal Atresia:

Duodenal atresia is the congenital absence or complete closure of a portion of the lumen of the duodenum.

Fetal Anomaly Ultrasound Scan:

A screening test offered to pregnant women to monitor the growth and development of the fetus before birth by producing a real time visual image. Scans before 16 weeks are useful for dating and assessing viability if the pregnancy (and are able to detect some major malformations). Detailed scanning at 18 – 21 weeks should show up most malformations as well as some minor ones.

False Positive:

Screening tests divide people into lower and higher risk groups. Some people with a positive screening test result do not actually have the condition being screened for. These people are said to have a false positive result.

Gastroschisis:

Developmental abnormality, which occurs as an abdominal wall defect associated with evisceration of the intestine.

Haemophilia:

Haemophilia is a group of hereditary genetic disorders that impair the body’s ability to control blood clotting or coagulation. Haemophilia A is the most common form. In this instance clotting factor VIII is absent. In haemophilia B factor IV is deficient. Haemophilia is an x linked chromosomal disorder.

Human Chorionic Gonadptrophin:

Originates from the chorionic villi and synchtiotrophoblastic cells of the placenta. Can be divided into three different types.Total, intact and free. Most commonly free beta hcg is the favoured one. Hcg can be measured in both the first and second trimester. Normally free beta hcg levels are raised in Down’s syndrome pregnancies.

Inhibin A:

Produced by the placenta. Most effectively measured between 15 – 20 weeks. Levels are affected by smoking thus the smoking status of the woman should be documented. Normally higher than normal results in pregnancies affected by Down’s syndrome.

Monoamniotic:

Identical twins that share a single amniotic sac.

Monochorionic:

Twins with a single placenta.

Naegeles Rule:

By counting forward 9 months and 7 days from the first day of the last menstrual period it is possible to arrive at an estimated date of delivery (EDD).

Nuchal Translucency Scan:

At 11 – 13+6 weeks of pregnancy the thickness of fluid at the nape of the fetal neck, the nuchal translucency, is measured. An increased amount of may indicate that the fetus has Down’s syndrome, structural or genetic anomaly. By combining the mother’s age and the gestation of the pregnancy with information from the scan an individual statistical chance of an anomaly can be given for that particular pregnancy. If the chance is one in 250 or higher a diagnostic test, such as CVS will be offered.

Oligohydramnios:

Amniotic fluid volume of less that 500mls.

Placenta Praevia:

Defined as a placenta that is partially or wholly implanted in the lower uterine segment on either the anterior or posterior wall.

Polyhydramnios:

Said to be present when the amniotic fluid volume in the deepest pool of amniotic fluid surrounding the fetus exceeds 8cm.

Pregnancy Associated Plasma Protein A PAPP-A:

Product of the placenta and is most effective for Down’s syndrome screening in the first trimester. Most effective time to measure is at 10 weeks gestation. Lower than normal levels are found in pregnancies affected with Down’s syndrome.

Pulmonary Hypoplasia:

Lungs are very stiff and difficult to inflate and there is usually a poor response to ventilation.

Quadruple Test:

Second trimester test to calculate the risk of Down’s syndrome, usually based on the measurement of AFP, uE3, free b- hcg (or total hcg) and Inhibin – A together with the woman’s age.

Soft Markers:

Changes noted on the ultrasound scan in the appearances of fetal physical structure. They cause no harm in themselves but can sometimes be associated with a more serious condition. Many markers have been identified for Down’s syndrome including absence of nasal bone or increased nuchal translucency.

Spina Bifida:

An anomaly where the spine has not closed over the central nervous system. If this lesion is at the head, the condition is called anencephaly and is incompatible with life. If it occurs lower down the spine it is called spinal bifida and results in varying degrees of physical and learning disabilities. Detailed ultrasound scanning is the way in which these conditions are confirmed, often following a raised AFP blood test. Most NTDs are “ open “ which means there is no skin over the lesion in the spine: about one in seven cases of spina bifida are “ closed “ which means that although the spine has not covered the nervous tissue there is a covering of skin. Closed conditions are less likely to be detected antenatally by the AFP test.

Triple Test:

Second trimester test normally taken at 16 – 18 weeks, usually based on the measurement of alpha feto protein (AFP) Unconjugated estriol (uE3) and hcg, which together with the woman’s age and gestation of the pregnancy, is used to calculate the risk of Down’s Syndrome.

Trisomy 18:

A syndrome caused by the presence of an extra copy (three instead of two) of chromosome 18. The combination of features present in babies affected with trisomy 18 can lead to many different problems including growth deficiency, feeding and breathing difficulties, developmental delays, learning difficulties, undescended testes in males, kidney malformations, heart defects. They may also have malformations in the bones. Survival of infants with trisomy 18 depends on how severely they are affected. Most do not survive the first year of life.

Trisomy 21:

Down’s Syndrome: A disorder caused by the presence of an extra copy (three instead of two) of chromosome 21. It affects all population groups and is distinguished by a number of features occurring together including low muscle tone, a face that appears flatter with eyes slanting upward, small ears and an unusually wide neck and deep crease across the palm of the hand. Some may have heart problems or visual problems or may develop Alzheimer’s disease. Although people with Down’s syndrome have learning difficulties, these vary in severity.

Unconjugated Estriol:

Product of the fetal adrenal glands. Most effectively measured from 14 – 22 weeks, lower than expected results found in Down’s syndrome pregnancies.

Zygosity:

Determining if twins, triplets are monozygotic (identical).

Zygote:

A zygote is a cell that is the result of fertilization. That is, two haploid cells — an ovum from a female and a sperm cell from a male — merge into a single diploid cell called the zygote (or zygocyte).