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 1-2%.
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 100 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.