Nuchal translucency scan or the first trimester scan is offered to women between the 12th and 14th week of pregnancy, and often, it is the first scan done to confirm pregnancy as well. While this scan is predominantly done to check the baby for Down’s syndrome, it also helps predict the estimated dates of delivery, therefore it is also called the dating scan.
Every mother carries some risk of having a baby with physical or mental illness, the risk increasing with maternal age. Tests such as these help achieve accurate results regarding fetal abnormalities by not depending on maternal age alone.
What Does The Test Measure?
This ultrasound measures the collection of fluid behind the baby’s neck, also called the nuchal fold. A small amount of fluid is present in every baby, but a greater accumulation of fluid indicates a greater risk for Down’s syndrome. Patients with Down’s syndrome often have circulatory disturbances. The accumulation of fluid in the nuchal fold indicates similar circulatory disturbances even in a fetus, thereby linking such a finding to Down’s syndrome.
What Is The Accepted Nuchal Thickness?
The average levels of fluid measured is around 2.18mm; however, about 13% of normal fetuses have shown a nuchal fold reading of 2.5mm. Hence for greater accuracy, this scan is combined with blood tests, which measure the levels of hormones – primarily hCG (Human Chorionic Gonadotropin) and PAPP-A (Pregnancy Associated Plasmaprotein-A). Mothers with a higher risk show increased HCG levels and decreased PAPP-A levels.
How Is The Test Performed?
The scan is performed by a sonographer, who smoothes a warm gel over the patient’s abdomen. The ultrasound machine is then gently run over the abdomen to scan the baby through the tummy. Sometimes, pictures might be a little unclear and in these cases a vaginal scan may be performed.
As mentioned earlier, the risk of Down’s increases greatly with maternal age. In cases when the nuchal scan reading is above normal, further diagnostic tests such as amniocentesis or chorionic villi sampling (CVS) will be recommended for a final diagnosis. These tests carry a slight chance of miscarriage and therefore one needs to discuss their options with their healthcare practitioners. CVS is performed in the 13th week and amniocentesis around 15th or 16th week.
Today, all leading hospitals and healthcare trusts are encouraging expectant mothers to undergo antenatal screening as this helps predict the health of a baby and relieve anxiety about baby’s welfare and how the mother is going to cope. However, some invasive tests such as amniocentesis and CVS carry a risk of miscarriage. Therefore, if termination of a pregnancy is not an option for a woman, she may opt out of having screening tests on the whole. Taking decisions regarding an unborn child is very hard, but your midwives and healthcare practitioners will be able to offer you counselling on your further course of actions.