Prenatal Screening

Prenatal screening

Screening for Down syndrome is available to about 53.5% of mothers on a maternal age basis, and the remaining 46.5% of health boards provide serum screening for all ages.

There are several methods used in prenatal screening, these are usually used separately, and a number of factors are taken into account to determine which method should be used. Amniocentesis has been around for 20 years and is probably the most well known screening method. It involves testing a sample of the amniotic fluid surrounding the foetus, ultrasound is used to guide a needle through the abdomen, into the womb and a small amount of amniotic fluid (20ml) is removed. The procedure is usually carried out at 14-16 weeks. Amniocentesis tests for chromosome disorders, and is 99.8% reliable for chromosome number, there is however a risk of miscarriage (usually 1/250 or less) after the procedure. This is one of the reasons why amniocentesis has only been offered to over 35's (since they have a much higher risk of having a Down syndrome child)(Webb 1990).

Previous studies on amniocentesis concentrated on problems that might arise during pregnancy or immediately after, these studies found that children whose mothers had amniocentesis are more likely to have breathing problems in the first few days after birth. A study performed by Jo-Anne Finegan in Toronto followed 88 women who had, had amniocentesis, there was an increased incidence of ear infection in this group. Finegan tested the stiffness of the eardrum and found children in the amniocentesis group were more than three times as likely to have abnormal readings. It is thought that there is a disruption of the delicate balance of pressure across the eardrum when the amniotic fluid is removed, which could cause the problems(Webb 1990).

Chorionic villus sampling is another form of sampling, it involves taking a small piece of placenta and genetic testing is carried out on it, there is a slightly higher chance of foetal loss with this procedure (Dick 1996).

A more recent form of prenatal testing involves serum markers. Blood is taken from the pregnant women and the maternal blood is tested for three hormones, this test is called the 'triple screen' test. The three hormones tested are alpha foeto-protein (AFP), human chorionic gonadotrophin (HCG) and oestradiol (E3). AFP is based on the fact that Down syndrome foetuses tend to be smaller on average, have smaller placentas and thus secrete less AFP.