The 11-13 weeks’ scan (performed between the beginning of the 11th to the end of the 13th week of the pregnancy)
Down syndrome (mongolism) is one of the commonest causes of mental retardation. The children with Down syndrome have always some degree of retardation and they commonly have other health problems, such as heart abnormalities. Any woman may deliver a baby affected with this condition but the risk increases as the mother gets older.
The only way to be absolutely sure whether the baby has Down syndrome is to perform an invasive procedure (chorionic villus sampling or amniocentesis) and examine genetic material from the placenta (chorionic villus sampling) or the amniotic fluid (amniocentesis), which is identical to the baby’s genetic make up. However these procedures have a small risk of miscarriage (about 0.5%). Therefore the decision on whether such a procedure is required is better to be based on the results of a test evaluating the risk of the baby having Down syndrome. This special test comprises of an ultrasound examination measuring the nuchal translucency and assessing the presence of the nasal bone, the blood flow in the heart (the tricuspid valve flow) and in a vessel in the abdomen (the ductus venosus flow). The ultrasound findings are combined with the measurement of two hormones in the mother’s blood the β-hCG and the PAPP-A.
The nuchal translucency is fluid present on the back of the baby’s neck, under the skin. All babies have some fluid but babies with Down syndrome have more than usual. Another significant part of the 11-13 weeks ultrasound scan is the examination of the fetal organs.
The combination of the ultrasound features and the hormones can detect about 9 out of 10 fetuses affected with Down syndrome (sensitivity of about 95% for 3% false positive rate). The test has equally good results for other chromosomal abnormalities such as trisomies 18 and 13 and Turner syndrome.
What does the result of the test mean?
The final result is a number representing your own personal risk for having a baby with Down syndrome. This number is based on your age, your history (it is important to inform the doctor if you had a previous pregnancy with a chromosomal abnormality or a family history of such problems), the baby’s measurements and the hormones in your blood. If for example the result shows a risk of 1 in 2000, this means that if there were 2000 women with this result one would have an affected baby and the remaining 1999 would have normal babies. The test cannot give you a 100% result, meaning that it cannot tell you for sure that the baby does or does not have Down syndrome.
This test for calculation of the risk for Down syndrome is very accurate and the results have been confirmed in hundreds of thousands of pregnancies worldwide. It is very important that the doctor performing the examination has received training and accreditation by the Fetal Medicine Foundation of London (FMF, www.fetalmedicine.com) that developed this method. The FMF audits the results of all accredited doctors on a regular basis to ensure high standard of care.
The results of the test will show a low risk for Down syndrome (ie a risk of less than 1 in 300) in most pregnancies. If your result shows a risk higher than that you should consult your doctor and decide whether you want to proceed to an invasive test (chorionic villus sampling or amniocentesis) to find out with certainty if the baby has a chromosomal abnormality like Down syndrome. However, even in the group that receives high-risk results, the majority of the babies will be normal. If for example the chance of Down syndrome is 1 in 100, this means there is a 99% chance that the baby will be normal.
Are there any other implications of having increased nuchal translucency?
The babies with increased nuchal fluid and normal chromosomes are usually normal. Nevertheless, they do have a higher risk for structural anomalies in particular heart problems as well as rare genetic conditions, than the general population. The majority of these problems can be diagnosed with special scans and other prenatal checks.
What else can the ultrasound examination at 11-13 weeks offer?
This examination allows for precise estimation of the gestational age, which is particularly valuable in women unsure of their dates or those with an irregular cycle. In multiple pregnancies the chorionicity (whether the babies share a common placenta or not) is determined and this is information is paramount in deciding the antenatal care protocol. Finally, this scan is the first detailed examination of the fetal organs and provides an opportunity to diagnose severe abnormalities. In fact almost half of the more serious fetal defects can be diagnosed at this early stage, provided that the examiner is adequately trained.