First Trimester Screening For Downs Syndrome
|Maternal age (yrs)||Chance of Downs Syndrome At 12 weeks||Chance of Downs Syndrome At birth|
|20||1 in 107||1 in 1530|
|25||1 in 950||1 in 1350|
|30||1 in 630||1 in 900|
|32||1 in 460||1 in 660|
|34||1 in 310||1 in 450|
|35||1 in 250||1 in 360|
|36||1 in 200||1 in 200|
|38||1 in 120||1 in 170|
|40||1 in 70||1 in 100|
|42||1 in 40||1 in 55|
|44||1 in 20||1 in 30|
Screening For Down’s Syndrome And Diagnostic Testing
The only way to be certain whether the fetus has Down’s syndrome or not is by doing a diagnostic test – an amniocentesis or chorionic villus sampling (CVS). A CVS is performed between 11 to14 weeks and involves taking a small amount of tissue from the placenta; an amniocentesis involves taking fluid from around the baby and is usually done after 15 weeks. Both provide a sample that contains tissue that has the same genetic make-up as the baby and will allow the baby’s chromosomes to be looked at in detail. But the problem with these invasive tests is that they can cause a miscarriage, even if the baby is entirely normal. The risk of miscarriage is about 1%.
Therefore the decision as to whether to have an invasive test or not must be taken on the basis of a non-invasive assessment of the likelihood (risk) that the baby has Down’s syndrome. The most accurate way of estimating this risk is by combining the information from:
- The age of the mother
- The findings from an ultrasound scan at 11 to 14 weeks (the nuchal translucency scan)
- Information obtained from the levels of certain pregnancy hormones in the mother’s blood.
The Nuchal Scan
The nuchal scan is an ultrasound scan performed between 11 and 13+6 weeks of pregnancy, during which the fluid at the back of the baby’s neck (the nuchal translucency) is measured. All babies will have some fluid here, but in babies with Down’s syndrome the fluid tends to be increased.
What else does the scan do?
Other things that we can do at the time of the scan are:
- Confirm your dates
- Check that the baby is developing normally
- Diagnose a multiple pregnancy
- Evaluate fetal anatomy that is developed and visible at this stage