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%.

The most accurate way of estimating the risk of the fetus having Downs 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
• Presence or absence of any physical abnormalities
• Level of two hormones (ß·hCG & PAPP·A) in the mother’s blood

Ultrasound picture of a 12-weeks fetus showing
the normal nuchal translucency and nasal bone

                                          http://www.nuchalscans.co.uk

This scan can almost always be carried out transabdominally.

There are a number of benefits:

1. 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.

2. To assess the risks of Downs Syndrome and other chromosomal abnormalities

Iindividual risk for pregnancy. This is calculated by taking into account the age of the mother, measurement of two hormones in the mothers blood and the scan findings of nuchal translucency thickness, nasal bone and fetal abnormalitites.

3. 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.

4. To diagnose certain major fetal abnormalities
Major abnormalities may be visible at this gestation but a 20 week anomaly scan is essential.

5. 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.