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Special Tests

Special Tests

There are a number of tests now which are offered in most maternity units to screen mothers for a variety of possible foetal abnormalities. Some of these tests do not tell you for certain that there is a problem. However if the test indicates that there is a high probability, you will be offered a diagnostic test to confirm or rule out the particular problem.

Nuchal Scan*

The risk of having a baby with Down Syndrome can be assessed around 11-13 weeks using a special ultrasound scan called a nuchal scan. (“Nuchal” means neck.) A shadow of a particular size and shape that is present at the back of the foetus neck may indicate a higher risk of chromosome defects such as Down Syndrome if it is thicker than normal in relation to the age of the mother. In this case, amniocentesis will be offered to confirm the diagnosis. In some centres, a blood test is performed with the nuchal scan to improve the detection rate of the test.

Serum Screening (Bart’s or Triple Test)*

This is a test that was developed by St. Bartholomew’s Hospital in London. A sample of the pregnant mother’s blood is taken at 16 weeks to measure the levels of three substances – oestriol, human chorionic gonadotrophin, and alpha-fetoprotein. The results can be assessed in relation to your age to predict the chance of your baby being born with Down Syndrome. If the chances seem high (more than 1 in 250), amniocentesis will be offered. The Bart’s triple test is not yet offered automatically at all centres, although you can request it.

AFP Test*

Alpha-fetoprotein is found in varying amounts in your blood throughout pregnancy. Between 16 and 18 weeks the levels are usually low, so if a blood test is performed at this time and the levels are 2-3 times higher than the average of a sample group, it may indicate a neurological problem such as spina bifida or hydrocephalus. However, these problems are nearly always diagnosed with greater accuracy by ultrasound, and the AFP test is being used less frequently. An abnormally low level of alpha-fetoprotein suggests that the foetus may have Down Syndrome and amniocentesis would be offered. However, the nuchal scan has superseded AFP testing in many units as a screening test for Down Syndrome.

Amniocentesis*

Amniotic fluid contains cells from the baby’s skin and other organs which can be used to diagnose his condition. Amniocentesis is the name given to the procedure that withdraws this fluid from the uterus.

Why is it done?

You may be offered an amniocentesis if you are over the age of 37, as the risk of chromosomal abnormalities (such as Down Syndrome) increases with age. You may also be offered it after serum screening (Bart’s test), or if a nuchal scan indicated a risk of Down Syndrome. In addition, amniocentesis can reveal other important information which may be sometimes helpful in determining the care and progress of your pregnancy.

Where there is cause for concern, the test may show the following:

  • The sex of the baby: cells sloughed off by the foetus accumulate in the amniotic fluid. Under the microscope, male cells can be distinguished from female cells and the baby’s sex ascertained. In gender-related genetic disorders such as haemophilia, a male child will have a 50 percent chance of being affected
  • The age of the foetus: if the lecithin/sphingomyelin (L/S) ratio in the fluid is measured, the maturity of the lungs can be assessed, which is in itself an indication of foetal age. However, this is rarely done now.
  • The chemical composition of the fluid: this can reveal metabolic disorders caused by missing or defective enzymes
  • The bilirubin content of the fluid: this helps to determine if a Rhesus-positive baby needs an intrauterine transfusion
  • The amount of oxygen the baby is getting: gases dissolved in the amniotic fluid can be measured, revealing whether the baby is at risk from lack of oxygen
  • The acidity of the fluid: this is another indication of foetal distress often caused by inadequate oxygen flow to the foetus
  • The chromosome count: determined by examining discarded cells. Any deviation from the normal chromosomal structure usually means that the baby may have a disability

Chorionic Villus Sampling (CVS)*

The chorion is the outermost of the two foetal membranes (the amnion is the innermost). Chorionic villi are finger-like outgrowths on the edge of the chorion and are genetically identical to the foetus. They develop earlier than amniotic fluid, so examining a sample of chorionic villi provides valuable information about your baby’s genes and chromosomes before amniocentesis is possible. However, CVS is only available at a few specialist centres, and is not as common as amniocentesis.

Umbilical Vein Sampling (Cordocentesis)*

This procedure is used to examine the constituents of foetal blood, and, in the case of foetal anaemia, for intrauterine blood transfusion. It is vital in four other situations:

Infection Detection

  • Rubella, toxoplasmosis, and the herpes virus may be detected by performing a specific radio analysis of certain proteins that are present in the blood of the foetus.

Rhesus Iso-Immunisation

  • In cases of Rhesus incompatibility, direct assessment of foetal haemoglobin is the best way to determine the severity of blood-cell destruction and whether an intrauterine blood transfusion (also done through the umbilical vein) needs to be carried out.

Suspected Growth Retardation

  • If the foetus is considered to be growth-retarded, cordocentesis will reveal the acidity or alkalinity of the blood, the amount of oxygen, the amount of carbon dioxide, and the amount of bicarbonate in the blood. In addition, plasma levels of glucose can be estimated.
  • Under ultrasonic control, a hollow needle is passed through the front wall of the abdomen and uterus into a blood vessel in the umbilical cord, about one centimetre from where it emerges from the placenta. A small quantity of blood can then be removed for testing. The risk to the foetus appears to be 1-2 percent. In theory, cordocentesis can replace any investigation currently undertaken on a blood sample.

Doppler Scan

  • This is a special scan, only available at a few centres. The Doppler scan looks at the flow of blood in the foetus or in the placenta. It is used when a foetus is small for dates or seems to not be growing as fast as it should. It uses a slightly different sort of soundwave from a normal ultrasound scan, which bounces off moving red blood cells, and indicates how fast they are moving through the foetus’s blood vessels.

Ultrasound Scan*

ultrasound

A routine ultrasound scan will reveal if your baby is healthy, and may be used at different stages of your pregnancy to:

  • Check the baby’s location and development in the placenta.
  • Check on the growth rate of the baby, particularly when the date of conception is unknown.
  • Discover whether the baby is ready to be born overdue.
  • Confirm your baby is in the usual head-down position, and not bottom-down, after week 38.
  • Detect certain foetal abnormalities, such as spina bifida.
  • Monitor the baby throughout special tests such as amniocentesis, and foetoscopy.
  • Assist in operations performed on the baby in the uterus.

Certain conditions may also necessitate extra scans. You may have extra scans:

  • As part of infertility assessment.
  • To identify abdominal problems such as ectopic pregnancy.
  • If the doctors suspect an imminent miscarriage.
  • To check for a multiple pregnancy.

For more information about Ultrasound scans visit Obstetric Ultrasound “A Comprehensive Guide".

* Dr Miriam Stoppard MD FRCP, Conception, Pregnancy & Birth, Dorling Kindersley, Australia 2002