11-14 Week Scan with Downs Syndrome Risk Assessment
The best time to perform this scan is at 13 weeks. The scan is usually done transabdominally, but occasionally it may be necessary to do the scan transvaginally:
The purpose of the 11-14 week scan:
- To date the pregnancy accurately. This is particularly relevant for women who cannot recall the date of their last period, have an irregular menstrual cycle, or who have conceived whilst breastfeeding or soon after stopping the pill. We measure the size of the fetus and from this we calculate the expected date of delivery.
- To diagnose multiple pregnancy. Approximately 2% of natural conceptions and 10% of assisted conceptions result in multiple pregnancy. Ultrasound scanning can determine if both babies are developing normally and if the babies share the same placenta which can lead to problems in the pregnancy. In such cases it would be advisable to monitor the pregnancy more closely.
- To diagnose certain major fetal abnormalities. Some major abnormalities may be visible at this gestation, but a follow up scan at 20-22 weeks remains essential. Not all anomalies are detected at 11-14 weeks.
- Unfortunately, in 2% of women who attend our clinic for the 11-14 week scan it is found that the fetus has demised, sometimes several weeks before and without any warning.
- Risk assessment for Downs syndrome and other more rare chromosomal abnormalities (Edward syndrome, Patau syndrome, Turner Syndrome and Triploidy) with nuchal translucency measurement.
- It is important to remember that 98% of all babies are normal.
Downs syndrome risk assessment for each individual patient
What is Downs Syndrome?
The term ‘syndrome’ is used to describe a collection of features which are often seen together. Down syndrome was first identified by Dr John Langdon Down in 1866, who noticed a similarity in appearance in some of his patients. These individuals possessed a broad, flat face, a thick tongue and a small nose and were intellectually impaired to a variable degree.
However, there are more differences than similarities between people with Down syndrome. Many babies with the condition will have one or all of the following features at birth: low muscle tone (a floppy baby), a face that appears flatter with eyes slanting upward, small ears and a wider neck than usual, a crease across the palm of the hand and a gap between the toes. Some may have heart problems. While intellectual disability is a feature of the syndrome, those with the condition will develop and learn throughout life, but at a slower pace than usual.
There is no cure for a child born with this condition but many symptoms can be treated and special early intervention programs are enabling these individuals to develop their potential. A child with Down syndrome can usually do most things that any young child can do such as walking, talking, dressing and being toilet trained although they may do these things later than other children.
In each human cell, except for the egg and sperm cells, there are 46 chromosomes, made up of 23 pairs. The chromosomes are numbered according to their size. When egg and sperm cells are formed the number of chromosomes they contain is halved to 23 with only one copy of each pair. A baby is formed when the sperm from the father fertilises the egg from the mother. The baby will now have 46 chromosomes, just like the parents, with one copy from each parent.
Sometimes, when the egg and sperm are forming, a mistake occurs so that the chromosome pairs do not separate in an ordered fashion. The result is an egg or sperm cell that has only 22 chromosomes while others have 24 chromosomes. If an egg or sperm carrying 24 chromosomes combines with an egg or sperm carrying the usual 23 chromosomes, the result will be an individual with cells in which there are 47 chromosomes instead of the usual 46. Thus there will be three copies of a particular chromosome in the cells rather than two.
Down syndrome is caused by an extra copy of chromosome number 21. For this reason it is also known as Trisomy 21. The presence of the extra chromosome causes the mental and physical characteristics of Down syndrome. In 95% of cases the extra chromosome is present in all the cells of the baby. In about 1% of cases, some of the baby’s cells will contain the usual chromosome number of 46 and others will contain the extra chromosome 21. This situation is known as ‘mosaic’ Trisomy 21. In about 4% of cases, the extra copy of chromosome 21 is attached (translocated) to another chromosome. This is called ‘translocation’ Trisomy 21. This type of Downs syndrome can be inherited. So where there is a previous child with Down syndrome or a close family history of Down syndrome, a discussion with a genetic counsellor can provide information about the risk of having an affected child and the availability of testing.
Who is at risk of having a child with Downs Syndrome?
The extra chromosome 21 can come from either the egg or the sperm. However the chance of having a baby with Down syndrome increases with a woman’s age. Table 1 shows the age-related risks of chromosomal problems occurring in a pregnancy. It is estimated that 1 in 3-4 fertilised eggs are chromosomally abnormal and this increases with the mother’s age. Therefore, most people have had at some time, a chromosomally abnormal conception which may have miscarried or not even been recognised as a pregnancy because the miscarriage occurred so early.
Table 1:Approximate risks of chromosomal changes associated with maternal age
|Maternal age at delivery||Chance of having a live-born baby with a Down Syndrome||Chance of having a live-born baby with a chromosomal abnormality|
|20-24 years||1 in 1478||1 in 506|
|30 years||1 in 909||1 in 385|
|31 years||1 in 900||1 in 385|
|32 years||1 in 750||1 in 323|
|33 years||1 in 625||1 in 286|
|34 years||1 in 500||1 in 244|
|35 years||1 in 384||1 in 179|
|36 years||1 in 307||1 in 149|
|37 years||1 in 242||1 in 124|
|38 years||1 in 189||1 in 105|
|39 years||1 in 146||1 in 81|
|40 years||1 in 112||1 in 64|
|41 years||1 in 85||1 in 49|
|42 years||1 in 65||1 in 39|
|43 years||1 in 49||1 in 31|
|44 years||1 in 37||1 in 24|
|45 years||1 in 28||1 in 19|
The only way to know for sure whether or not the fetus has a chromosomal abnormality is by having an invasive test such as chorionic villus sampling (CVS) or amniocentesis (amnio). However, these tests carry a risk of miscarriage of about 0.3% (1 in 300 chance) for our unit.
It is up to you and your partner to decide whether or not the risk of the fetus having a chromosomal abnormality is high enough to warrant having an invasive test. As a guideline, an invasive test is usually offered if the risk of Down’s Syndrome is 1 in 300 or above.
The most accurate way of estimating the risk of the fetus having Down’s Syndrome is carried out at 11-13 weeks and depends on the:
- Age of the mother.
- Amount of fluid behind the neck of the fetus (nuchal translucency).
- Presence or absence of the fetal nasal bone.
- Fetal heart rate.
- Blood flow through the tricuspid valve of the fetal heart.
- Blood flow through the ductus venosus in the fetal liver.
- Presence or absence of any physical abnormalities.
- Level of two hormones (free ß-hCG and PAPP-A) in the mother’s blood. The blood test needs to be done before the scan and if possible at 9-10 weeks.
After the scan, on the basis of all the above factors, the estimated risk for Down’s Syndrome will be discussed with you. Only you can then decide if you wish to have an invasive diagnostic test.
Irrespective of whether or not you decide to have an invasive test, it is recommended that you have a scan at 20 weeks to check for physical abnormalities.