Obsterics

The "Triple" test

A screening test for Trisomy 21 (Down"s syndrome), other trisomies and spina bifida.

This test is performed between 11 and 14 weeks after your last menstrual period and is offered routinely to all pregnant women at St Luc. Before agreeing to the test it is important to decide if you would like to know the result, if you would be willing to undergo an invasive test such as amniocentesis if you are identified as being in a high risk group and what you would do if your baby was found to be affected. This can be discussed in full with your obstetrician before the test if you have any outstanding questions.

An ultrasound scan of your baby is performed in order to measure the skin thickness at the back of the neck. This measurement is then combined with the levels of two proteins in your blood (via a blood sample) taking into account your age. The result is expressed as a probability of your baby being affected, for example 1 in 100 or 1 in 2000. From this test result you will be advised whether or not you are at high or low risk of having a baby affected by one of the conditions listed above. Unfortunately this test is unable to say categorically whether your baby is or is not affected. If you have a high risk result (typically 1 in 250 or worse, found in approximately 5% of pregnant women), then your obstetrician will discuss the possibility of performing an amniocentesis to obtain some liquid surrounding your baby that can be analysed to identify an abnormality (in the numbers of chromosomes). In the vast majority of cases (49/50), the result is normal. Unfortunately this procedure, even when carried out by trained personnel under ultrasound guidance, carries a risk of miscarriage of approximately 1 in 250. (Amniocenteses or in some cases placental biopsy (chorionic villous sampling), are also performed in the presence of abnormal ultrasound findings, inherited genetic diseases in the family or simply as a function of high maternal age).

The triple test does not identify all cases of abnormality but when combined with the routine use of detailed ultrasound scans it will identify more than 90% of cases.