Down’s syndrome is caused by the presence of an extra chromosome number 21 in the cells of the developing baby. About one in every 700 babies are born with Down’s syndrome. Usually it is not inherited and so a baby can be affected even if there is no history of Down’s syndrome in the family.

Down’s syndrome is the most common cause of severe mental handicap and is often associated with physical problems such as heart defects or difficulties with sight and hearing. It is not possible to access the degree of handicap before the baby is born. Nine out of ten babies with Down’s syndrome will survive their first year and nearly half of these will reach 60 years age.

What are open neural tube defects?

The two main kinds of neural tube defects (NTDs) are spina bifida and anencephaly. Babies with spina bifida have an opening in the bones of the spine which can result in damage to the nerves controlling the lower part of the body. This causes weakness and paralysis of the legs and sometimes bowel and bladder problems. Babies with spina bifida are also more likely to have a collection of fluid in the brain, called hydrocephalus, which can be treated surgically but can lead to mental handicap.

Babies with anencephaly have a large part of the skull missing and the brain is not properly formed. They always die before or very soon after they are born.

In about one in every five babies with spina bifida the spinal opening is covered with skin or thick tissue. This is called close spina bifida and will not be detected by blood test. This condition is usually less severe than open spina bifida.

What does the serum screening test involve?

A sample of the blood is taken between 15 and 20 weeks of pregnancy (16 – 18 weeks is the best time to the screen for open neural tube defects). The stage of pregnancy is the best estimated by an ultrasound dating scan. The levels of three substances in the blood will be measured and compared with the average levels for the same stage of pregnancy. The substances are:
1. AFP
2. uE3
3. β-hCG
The concentrations of these three substances are used with the maternal age to estimate the risk of Down’s syndrome in the pregnancy. The level of AFP is also used to determine if there is an increased risk of spina bifida or anencephaly.

What is the risk?

A risk is the chance of an event occurring. For example: a risk of Down’s syndrome of 1 in 100 means if 100 women have this test result, we would expect that one of these women would have a baby with Down’s syndrome and that 99 would not.  This is the same as a 1% chance that the baby has Down’s syndrome and 99% chance that the baby does not.

What does a screen negative result mean?

If the risk of Down’s syndrome, based on the maternal age and the levels of the three blood markers, is lower than 1 in 270 and AFP level is not high, then the result is called Screen-Negative and a diagnostic test would not be offered.

What does a screen positive result mean?

A screen positive result means that you are in a higher risk group for having a baby with Down’s syndrome or a neural tube defect. If your result is in this group, you will be offered a diagnostic test.
The result is called screen-positive if

1. The risk of Down’s syndrome in your pregnancy is 1 in 270 or greater. About 1 in every 20 women screened will be in this group. Or
2. The AFP level is more than two and a half times higher than the normal level.
About 1 in every 40 women screened will be in this risk group.
Most women with screen-positive results do not have a pregnancy with Down’s syndrome or a neural tube defect. For example : Of 50 women with screen positive results for Down’s syndrome, only one would actually have a pregnancy with Down’s syndrome.

Can any other abnormality be identified?

The risk of Trisomy 18 (a rare and a usually fatal abnormality) can be estimated using AFP, uE3 and β-hCG. In cases where risk is high this is reported.

Why do you take age into account?

Any women could have a baby with Down’s syndrome, whatever her age, but likelihood of this happening does increase as a woman gets older and so we use age as one of the factors when working your risk of pregnancy with Down’s syndrome. It also means that an older woman is more likely to have a result in the higher risk group (screen-positive) and so be offered a diagnostic test.