Nuchal Thickness Screening
Nuchal thickness screening is an ultrasound technique for calculating the chance of a baby having a chromosome problem.
The vast majority of babies are normal. However all women, whatever their age, have a small risk of delivering a baby with a physical and/or mental handicap.
In some cases the handicap is due to chromosomal abnormality such as Downs Syndrome.
The only way to know for sure whether or not the fetus has a chromosomal abnormality is by having an invasive test such as chorion villus sampling (CVS) or amniocentesis (amnio).
However these tests carry a risk of miscarriage of about 1%.
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 Downs Syndrome is 1 in 300 or above.
The most accurate way of estimating the risk of the fetus having Downs Syndrome is carried out at 11-13+6 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
- Presence or absence of any physical abnormalities
|
Above: 10 minute clip of full fetal evaluation including anatomy, NT measurement, fetus drinking movements and 3D images |
Ultrasound picture of a 12-weeks fetus showing the normal nuchal translucency and nasal bone.
After the scan, on the basis of all the above factors, the estimated risk for Downs 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.
The nuchal thickness scan can almost always be carried out with an abdominal scan. There are a number of benefits:
- To date the pregnancy accurately. This is particularly relevant for women who cannot recall the date of their last period, have an irregular 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 assess the risks of Downs Syndrome and other chromosomal abnormalities. Each woman will be given an estimate of her individual risk for this pregnancy. This is calculated by taking into account the age of the mother, the scan findings of nuchal translucency thickness, nasal bone and fetal abnormalities. Parents will receive full counselling concerning the significance of these risks and the various options for further testing.
- 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. Major abnormalities may be visible at this gestation but a 20 week anomaly scan is essential.
- To diagnose early pregnancy failure. Unfortunately, in about 3% of women who attend for a nuchal scan it is found that the fetus has died, often several weeks before and without any warning. Couples will receive full counselling as to the possible causes of this problem and the options for subsequent measures that may be necessary.
|
Superior combined NT screening.. A wide variety and combination of screening tests are now available. Blood tests at 10-14 weeks (1st trimester) and 15-20 weeks (2nd trimester), ultrasound tests at 11-14 weeks and later in pregnancy. Most test results can be combined to improve detection rates. Certain combinations of test have specific names; nuchal thickness screening combined with 1st trimester double biochemistry is referred to as the ‘combined test’; combined 1st and 2nd trimester biochemistry is referred to as the ‘integrated test’. How well do the tests work? The ‘sensitivity’ of a screening test describes the proportion of Down syndrome fetuses correctly identified by that screening test as being at ‘high risk’ of having that problem. The ‘false positive rate’ describes the proportion of all pregnancies screened that end up in the ‘high risk' group. The ideal test has a high sensitivity and a low false positive rate. Basic NT screening is the best single test and Mr Selinger, Jane Smith and Thea Warwick at Berkshire Independent Scanning all have many years (and thousands of cases) experience. Recent audit data suggests a sensitivity of 95% and a false positive rate of less than 5%. We are introducing improvements to our ultrasound examination so that nose bone presence, ductus venosus flow, tricuspid valve function and facial profile evaluation are included in the risk calculation. We can also arrange for the combined test to be completed. We are also able to offer combined NT + 1st trimester biochemistry by Genome plc or the NHS and NT, 1st trimester and 2nd trimester biochemistry by Genome plc. Genome is the country's foremost biochemistry screening laboratory run by Professor Cuckle in Leeds, previously named the Leeds Antenatal Screening Service. |
Below: All 4 children Scanned By Jane Smith
|