Trisomy 21 tests only manage to detect 50% of Down’s syndrome cases. For Dr. Jean-Martin Baillargeon, a research radiologist at Saint-Luc Hospital in the CHUM system, this success rate is still too low. “By doing an ultrasound in the first trimester, i.e., in the 12th week of pregnancy, we can increase the rate of detection of fetal anomalies considerably,” he insists, referring to a study conducted in the United Kingdom in 1998 by Kypros Nicolaides on nearly 100,000 patients.

At the present time, the only method used in Québec hospitals to detect Down’s syndrome is ultrasound in the second trimester, between the 18th and 20th week of pregnancy. For women over 35 years of age, amniocentesis is also prescribed. This nuchal translucency test, conducted at the time of the first trimester ultrasound, is especially effective in detecting Down’s syndrome. “On the screen, we can measure the liquid in the space between the muscles and skin in the neck of the foetus. Obviously, there is some space in all babies. But in babies at risk of Down’s syndrome or even trisomy 18, it is thicker.” To be valid, this test must be done between the 12th and 14th months of pregnancy. However, this by no means signifies that the second trimester ultrasound should be eliminated. “There are features that can only be detected in the second trimester—some cardiac malformations, for example. As a result, it is essential to do two ultrasounds: the first at 12 weeks and a second between the 18th and 20th week.”

For the past ten years, French and British women have had access to an early examination, then a second one in the 22nd week and even a third in the 32nd week. But in Québec, the only routine ultrasound is still the one in the second trimester. “If they ask their doctor, women can have an ultrasound in the first trimester. However, this examination is not prescribed systematically. With time, we hope that things will change and that all women, regardless of their age, will have both ultrasounds.”

During pregnancy, about 75% of cases of Down’s syndrome are detected by the nuchal translucency test. These results could be reinforced by a blood test to measure two proteins associated with pregnancy, PAPPA and beta-HCG. But before a pregnancy is actually interrupted, it is essential that amniocentesis be done. “Ultrasound in the first trimester and the normal blood test are only indicators. Amniocentesis, meanwhile, can determine the genetic makeup and give 100% reliable results.”

In Québec, three out of four children with Down’s syndrome are born to mothers under 35 years of age. The nuchal translucency test can determine whether a 38 year-old woman has a 1 in 1000 risk of carrying a Down’s syndrome baby. In this case she can go without amniocentesis. However, a 30 year-old woman may present a 1 in 100 risk, that is, a risk generally associated with a 40 year old woman. She may then feel that it is worth having amniocentesis.

Researcher: Jean-Martin Baillargeon
Telephone: (514) 733-4757