Antenatal Care Screening for Downs Syndrome

Down’s syndrome is a condition in which an extra copy of all or part of chromosome 21 is present. It causes cognitive impairment, poor motor skills, heart defects, and a characteristic facial appearance. Down’s syndrome is more common in older mothers and can be screened for during pregnancy. There are a number of different screening methods available for expectant mothers, which can be performed at different stages of pregnancy.

First trimester combined test

This is the earliest screening method and is performed between weeks 11 and 13 of the pregnancy. It involves a nuchal translucency screening test where ultrasound is used to detect fluid build up in the baby’s neck, and a blood test to measure levels of human chorionic gonadotropin and pregnancy-associated plasma protein A. The disadvantage of this early risk assessment is that approximately 5% of tests result in a false positive which results not only in causing unnecessary stress to the parents but also in further invasive testing which may put the baby at risk.

Full integrated test

This is similar to the combined test described above, but is performed over a longer period of time and results in fewer false positives (1%). During the first trimester the nuchal translucency test is done as well as a blood test to determine pregnancy-associated plasma protein A levels. Later in the pregnancy, at around 15 to 20 weeks, further blood tests are performed measuring levels of human chorionic gonadotrophin, estriol, alpha-fetoprotein and inhibin A. These blood tests are also known as a Quad screen.

A positive result in either the combined or integrated test indicates a high risk of the baby having Down’s syndrome. Only around 5% of these high risk babies actually have Down’s syndrome however and more invasive testing, which has an accuracy of around 98-99%, is required to determine if the baby has the condition.

Amniocentesis

This is performed at around 15 weeks and involves testing a sample of amniotic fluid. The fluid is removed from the mother’s uterus via a needle inserted through the abdomen. The insertion is guided by ultrasound to minimise the risk of harming the baby, and the procedure takes place under local anaesthetic. Some fetal cells are present in the amniotic fluid and the chromosome content of these can be analysed. The test does carry a risk to the baby however, with approximately 0.2-0.5% of tests resulting in a miscarriage.

Chorionic villus sampling

Chorionic villus sampling (CVS) involves removing some cells from the placenta either via a needle inserted into the abdomen as in amniocentesis, or by threading a tube through the cervix. Again the procedure using ultrasound as a guide and takes place under local anaesthetic. The cells removed are used to examine the baby’s chromosomes. CVS can be performed earlier than amniocentesis, usually taking place between the 9th and 14th weeks, but does have a higher risk associated with it. Approximately 1% of CVS results in the mother miscarrying the baby, however the advantages of an earlier diagnosis may make this risk worthwhile to the parents

Percutaneous umbilical blood sampling

This tests cells present in blood removed from a vein in the umbilical cord to determine the presence of a chromosomal defect in the baby. The procedure is similar to those described above. It has a greater risk of miscarriage associated with testing than either amniocentesis or CVS, however the results are obtained more quickly. This test is therefore usually performed after week 18 of the pregnancy when it is important to determine whether the baby has Down’s syndrome as soon as possible, particularly if the parents may be considering terminating the pregnancy if the child is affected.

These are the various antenatal screening techniques available to determine if a baby has Down’s syndrome. It is best to discuss which, if any, are suitable for you with your doctor, who can also answer any questions you may have and go over the associated risks and benefits in detail.