Screening Tests

a) Maternal Serum Screening (MSS)

This is a blood test carried out at about 16 weeks of pregnancy. A small amount of blood is taken from the mother and the levels of certain hormones and proteins are tested. These are known as markers. There are minimal risks to mother or baby from the procedure.

This test may be referred to as the double, triple or quadruple test depending on which markers are included.

The test measures the levels of the markers along with the mother's age, weight and exact gestation to assess the risk of having a child with a condition. For example, high levels of AFP indicate the presence of a neural tube defect such as spina bifida. Low levels of AFP and higher levels of hCG indicate the possible presence of Down's syndrome.

A screening test does not give a definite result, it will only give a risk estimation. Pregnancies with a risk of greater than 1 in 250 will be offered a diagnostic test.

View statement on the reliability of the serum screening process.

b) Ultrasound

Ultrasound scanning uses sound waves to form a picture of the baby in the uterus. It is completely painless and can be carried out at any stage of pregnancy. There is no evidence that ultrasound carries any risk to mother or baby.

An ultrasound scan is usually carried out at about 11 to 13 weeks of pregnancy to confirm the age of the baby and to determine the presence of twins. It is also an early opportunity for screening. In some parts of the UK, the radiographer will make a measurement of nuchal translucency, the amount of fluid around the back of the baby's neck. If this is increased it indicates a risk of chromosomal abnormalities. Combined with other factors - such as age of the mother - this measurement allows a more precise calculation of the chance that the baby will be affected by Down's syndrome. Early results of this new procedure suggest that 80% of all Down's babies can be detected by using nuchal translucency combined with other factors.

A detailed ultrasound scan may be offered at around 20 weeks of pregnancy. This scan checks to see if the baby is growing properly and where the placenta (afterbirth) is positioned. This scan also looks at other features of the baby such as the face, heart, kidneys and the brain.

Some abnormalities can be detected and diagnosed immediately. If the baby has spina bifida, an abnormality of the spine, this will usually be detected at this scan. In other words, ultrasound can be a diagnostic test for conditions which involve structural differences in the baby, such as spina bifida, missing limbs or faulty organs.

However, sometimes ultrasound only gives clues as to possible abnormalities. In these cases, women are told that their baby is at higher risk of the abnormality. An amniocentesis may be offered. Dealing with unexpected information, or uncertain information, can be very stressful.