What is a Nuchal Scan?
It is an ultrasound scan that is carried out from 11 weeks to 13 weeks + 6 days. The scan is usually performed transabdominally but in a few cases, it may be necessary to do the examination transvaginally. This is one of the most important scans during pregnancy and is recommended for all women.
What Are the 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 exactly and have an irregular menstrual cycle or have conceived while breastfeeding or soon after stopping the contraceptive pill. We measure the size of the baby (CRL, crown to rump length) and calculate the gestational age and expected date of delivery accordingly.
- To diagnose multiple pregnancies: Approximately 2% of natural conceptions and 10% of assisted conceptions result in multiple pregnancies. Ultrasound scan can determine if both babies are developing normally and if the babies share the same placenta which can lead to some problems in the pregnancy. In such cases, we will have to monitor your 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 miscarriage: Unfortunately, in 2% of women who attend for a nuchal scan, it is found that the baby has died, often several weeks before and without any symptoms. Couples will receive full counselling and workup as to the possible causes of this problem and the options for subsequent measures that may be necessary.
- To assess the chance of Down syndrome and other chromosomal conditions: A baby with genetic or chromosomal problem can show variations that may be visible on ultrasound which can help us decide the need for further testing.
- This scan along with a blood test (Double Marker Test) in the mother can help us check her chance of having a baby with Down’s Syndrome, Edwards’ Syndrome, and Patau’s Syndrome. This is calculated by taking into account the age of the mother, fetal heart rate measurement, measurement of two hormones in the mother’s blood (free ß-hCG and PAPP-A), and the scan findings of nuchal translucency thickness (measurement of the fluid present behind the baby’s neck), nasal bone, blood flow through the fetal heart and ductus venous and presence of any fetal abnormalities. The nuchal scan along with the blood test has a sensitivity of 90-95% for detecting Down syndrome and other chromosomal abnormalities.
- Parents are informed about the results as soon as they are available and are given detailed counseling concerning the significance of the results and the various options for further investigations including invasive testing or the NIPT.
What if My Nuchal Translucency Scan Result Shows an Increased Chance of the Presence of a Chromosomal Condition?
The nuchal translucency scan is a screening test which means that this test can only give a probability for a condition being present. It is not a diagnostic test and thereby it cannot confirm if a chromosomal condition is present or absent. If your nuchal scan screening shows an increased chance, then we will provide you with recommendations for further testing (such as a CVS or amniocentesis) to confirm if the baby actually has the problem.
What Is the Next Scan That I Should Have After the Nuchal Scan?
If your nuchal scan screening shows a low probability, then the next scan that is recommended is the anomaly scan at 18-22 weeks of pregnancy.