This scan is carried out from 11 weeks to 13 weeks and six days. The scan is usually performed transabdominally but in a few cases it may be necessary to do the examination transvaginally.
Aims of the nuchal 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 major fetal abnormalities. Some major abnormalities may be visible at this gestation. However it will still be necessary to have a 20 week anomaly scan.
- To diagnose early miscariage. Unfortunately, in 2% 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.
- To assess the chance of Down's syndrome and other chromosomal abnormalities. Each woman will be given an estimate of her individual chance for this 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, blood flow through the fetal heart and ductus venosus and fetal abnormalitites. Parents will receive full counselling concerning the significance of these chances and the various options for further investigations including invasive testing or the Harmony test.