Down’s Syndrome Screening (Nuchal scan)
OMNI offers Down’s syndrome screening to women of all ages (above and below 35 years)
The vast majority of babies are born normal. However, all women have a small risk of delivering a baby with Down’s syndrome. This is the most common chromosomal abnormality known. Down’s syndrome screening or the Nuchal Translucency (NT) scan is a non-invasive way of determining if the pregnancy is low or high-risk for Down’s Syndrome. This is a screening test which does not harm the baby and estimates the risk of your baby having Down’s syndrome. You and your partner can then decide if the risk of your fetus having a chromosomal abnormality warrants having an invasive test such as chorionic villus sampling (CVS) or amniocentesis. These invasive tests will give you a definitive answer, but also carry a small risk of miscarriage (0.5 – 1.0%).
The Nuchal Translucency Scan
The main part of the NT test involves an ultrasound scan. You should drink normally before arriving but do not empty your bladder 30 minutes before the scan. The scan is usually done through your tummy, but depending on the position of your baby and your womb, occasionally it is necessary to perform a vaginal scan, which will give better views. There is no risk to you or your baby and it should not be too uncomfortable. The NT is a collection of fluid under the skin at the back of a baby’s neck that can be measured using ultrasound. All babies have some fluid, but in many babies with Down’s syndrome, the NT is increased. NT scans are usually performed between 11 weeks and 13 weeks + 6 days gestation.
OMNI sonographers are accredited with the Fetal Medicine Foundation to perform NT scans. We will examine the skull and brain, the nasal bone, the arms, the legs, the stomach, the spine, the abdomen and the bladder. During the NT scan, several measurements are taken:
- The crown rump length (CRL) is the distance from the top of baby’s head to the bottom of the spine. This will enable us to accurately calculate how pregnant you are and therefore know the estimated date of delivery
- The fetal heart rate will be measured
- The NT measurement. This is the fluid filled space behind the neck that is present in all fetuses
The accuracy of this test is increased by measuring the level of two placental hormones in the mother’s blood (free hCG & PAPP-A). These hormone levels should be measured between 10 weeks and 13 weeks gestation.
The risk of having a baby with Down’s syndrome is calculated by combining the maternal age, the thickness of the NT measurement, the length of the baby and the levels of two placental hormones in the mother’s blood (free hCG & PAPP-A).
The vast majority of women who undergo the combined NT/biochemical test have a low-risk for Down’s syndrome, i.e. a calculated risk less than 1 in 300. About one in 20 women will be given a high-risk, and the vast majority of these go on to have a normal baby. Even with a risk as high as 1:5, the baby has four out of five chances that he/she does NOT have Down’s syndrome. Nevertheless, should your pregnancy have been labelled high-risk, you will probably feel anxious and perhaps unsure of what to do.
The only way to know for certain if your baby has Down’s syndrome or another chromosomal abnormality is to have a diagnostic test such as CVS (Chorionic Villus Sampling) or amniocentesis. This decision can be very difficult, but you do not have to decide in a hurry. If you are unsure what to do, you can wait until 15 weeks and have an amniocentesis then if you wish. OMNI will provide more information if you need it.