Prenatal Screening for Down's Syndrome

My interest in Down's Syndrome testing was fueled by the BBC documentary ' A World Without Down's Syndrome ' by Sally Phillips who in fact has a son, Olly, with Down's Syndrome. In addition, my teacher suggested a radio programme called A Moral Maze where people with opposing opinions could discuss their thoughts on the new 'fool-proof' prenatal Down's tests.

What is Down's Syndrome?

Down's Syndrome is a genetic disease also known as Trisomy 21. Down’s syndrome is caused by an extra copy of chromosome 21 in every cell of a person’s body. Most of the time, it is not inherited and is the result of a random genetic mutation in the sperm or the egg. 

        

Down's Syndrome Testing:

It measures the nuchal translucency and two pregnancy hormones called Papp-A (pregnancy-associated plasma protein) and HcG (human chorionic gonadotrophin) and is performed between 11 and 13 weeks of pregnancy.

The nuchal translucency scan is an ultrasound scan which measures the pocket of fluid at the back of the baby’s neck. An increased amount of fluid may mean the baby has an increased chance of having Down’s syndrome. The measurement is combined with a blood test and age to estimate the risk.

For women who want to be screened after 13 weeks, they can have blood tests between 16 and 19 weeks. It can either be:

1. The triple test where three hormones are measured (Hcg, afp and uE3)

2. The quadruple test where four hormone levels are measured (uE3, afp, Hcg and inhibin)

Screening test should have a detection rate of at least 75%. The likelihood that a pregnant woman will be given a false positive is less than 3%. This is when a woman is told that her baby has Down’s when in fact it does not. 

If screening shows you have an increased chance of having a baby with Down’s syndrome, you will be offered one of two diagnostic tests.

These are called CVS or amniocentesis.

Amniocentesis: A sample of the amniotic fluid surrounding the fetus is withdrawn through a needle inserted into the mother's uterus.

This sample is then used to analyse the chromosomes of the fetus. Doctors usually perform this test in the second trimester, after 15 weeks of pregnancy. The test carries a slight risk of miscarriage, but risk increases if it's done before 15 weeks. 

Chorionic villus sampling (CVS): cells are taken from the placenta and used to analyse the fetal chromosomes. Typically performed in the first trimester, after 10 weeks of pregnancy, this test appears to carry a somewhat higher risk of miscarriage than second trimester amniocentesis.