All pregnant women should consider having this screening test at the start of their pregnancy as it will help to reassure the vast majority of women that their baby appears to be developing normally. It will also detect at an early stage, those babies that might have a problem. This becomes increasingly important as women get older because their chance of having a baby with a chromosomal condition such as Down’s syndrome will become higher.
When is the screening test carried out?
The test is performed between 11 weeks and 13 weeks 6 days into your pregnancy. It cannot be performed outside this time frame as certain features that need to be seen on the scan are not present. If you are unsure of your period dates or have an irregular cycle, we suggest that you contact your midwife or GP as early as possible to arrange for a dating scan at your local hospital.
What does the test involve?
Using a combination of ultrasound and the measurement of hormones in your blood the risk of your baby having Down’s syndrome can be predicted. The ultrasound scan can also examine your baby in some detail to check whether many of the main organs are developing normally.
What does the scan examine?
The ultrasound scan measures the fluid within the skin at the back of the baby’s neck. This is known as the nuchal translucency. This is present within all babies, but the wider the nuchal translucency measures, the higher the change of your baby having Down’s syndrome. Equally the narrower the nuchal translucency measures, the lower the chance.
The scan will also check whether your baby has a visible nasal bone. In the past few years it has been discovered that approximately 3 in 4 babies with Down’s syndrome do not have a visible nasal bone at the time of the first trimester screening test. If the nasal bone is visible at the scan then this will reduce the chance of your baby having Down’s syndrome. Occasionally the nasal bone cannot be seen at 11 weeks as it is too early in the pregnancy. If this is the case and this creates some concern then the scan can be repeated a week later. However if the nasal bone cannot be seen clearly but the nuchal translucency and blood tests are predicting a low risk result it is sometimes not necessary to see the nasal bone in every case.
The ultrasound can also check whether your baby has all its limbs, that the head and brain appear to be developing normally, that the baby has a visible stomach and bladder and the umbilical cord is inserted correctly. From 12 weeks the spine can usually be seen clearly enough to rule out major cases of spina bifida. All of this information will hopefully provide important reassurance.
The scan cannot tell you the sex of the baby as all babies at this stage appear similar. The earliest we could sex the baby with any degree of certainty would be 18 weeks and even then we could not guarantee.
What is the blood test for?
The blood test taken before your scan is used to measure two hormones in your blood. They are both produced by the placenta and are unique to each pregnancy. If the hormone levels are favourable they will reduce the chance of your baby having Down’s syndrome. If they are less favourable they will increase the chance. Adding in the blood test increases the overall accuracy of the test.
Ideally it is better if you can have the blood test at least one day before the scan so that we have the results back when you come for the scan. The overall risk can then be calculated immediately after the scan and the result discussed face to face. If you cannot get the blood test done before the scan it can be taken on the same day but the result will need to be discussed the following day on the telephone.
How is the overall risk calculated?
The starting point for the calculation of risk is your age at the time your baby is due. Thus the older you are then the higher the starting risk of your baby having Down’s syndrome will be. By combining the results of the scan and the results of the blood test with your risk based upon your age the computer will calculate the overall risk of your baby having Down’s syndrome.
How accurate is the test?
The risk that you will be given will not tell you for certain if your baby has Down’s syndrome. However if the overall risk shows that the chance of your baby having Down’s syndrome is in the thousands then the accuracy of the test is very good and most people would feel reassured. If the overall result shows the chance is in the tens, again the accuracy is very good, but there is a significant chance your baby has Down’s syndrome. You may then consider having an invasive test. If you overall risk is around the 1 in 300 region then the detection rate of the test falls and you have to make a difficult decision whether to proceed to a further test or not. Thankfully for the majority of people the test performs in such a way that they are given a clear indication as to which way they should proceed. In other words if your baby does not have Down’s syndrome then the test will almost tell you the answer in around 90% of cases. This is much clearer than using a blood test combined with your age or just relying upon your age as an indicator.
What happens if I get a higher risk than I am comfortable with?
Most results will be extremely reassuring and will hopefully allow you to go a long way to answer that important question ‘is my baby normal’. A few results will be ‘borderline’ and will require discussion. Equally a few will be non-reassuring. In that case you may wish to consider an invasive test such as amniocentesis of chorionic villous sampling (CVS).
CVS can be performed after 11 weeks gestation and involves removing a small piece of placental tissue using a needle through the wall of the abdomen. It carries a risk of miscarriage of around 1%.
Amniocentesis can be performed after 15 weeks gestation and involves removing a small amount of amniotic fluid using a needle through the wall of the abdomen. It carries a risk of miscarriage of around 0.5-1.0%.
Can an increased nuchal translucency indicate anything else?
If the nuchal translucency is wider than expected other conditions may be present such as a heart condition. This will be discussed with you further if the thickness is greater than 3mm. In this case it would be recommended that you have a detailed cardiac scan at around 18-20 weeks with a careful review and discussion by consultants experienced in fetal medicine.
©2005 Babyvision Limited
Jan 2007 |
