An ultrasound, also know as a sonogram, is a test done during pregnancy which uses sound waves to generate a picture or image of the fetus. Occasionally, but not always, infants with Down syndrome show subtle signs on an ultrasound that can make the doctor suspect prenatally that the fetus has Down's syndrome.
Screening Indicators for Down's syndrome-
This ultrasound test is performed between 11 and 14 weeks of pregnancy to measure the clear space in the folds of tissue behind a developing baby's neck. In babies with Down syndrome and other chromosomal abnormalities, fluid tends to accumulate here, making the space appear enlarged.
Increased nuchal translucency or a measurement greater than 3 mm indicates that the risk of some genetic disorders and birth defects, including Down syndrome, is increased. Down's syndrome is accurately indicated in about 80% of cases.
Brachycephaly is a smallness of the head. In ultrasound terms it is the shortening of the occipitofrontal diameter (front to back of head) of the fetal head. In 15% of cases Downs is indicated as compared to normal fetuses.
Enlarged ventricles in the fetal brain may be associated with a higher incidence of chromosomal anomalies. The presence of even mild ventriculomegaly is a strong indicator to go in for further testing for Down's syndrome.
An enlarged tongue of the baby in gestational ultrasounds is also an indicator. Progressive enlargement and protrusion of the tongue of the foetus as pregnancy advances can sometimes be an indicator. Very rarely a flattening of the facial features may be seen on ultrasound.
Enlargements of the kidneys and renal system, renal anomalies or deviations from the normal renal anatomy have also been identified as markers for Down syndrome.
Shortening of the limbs:
There is an an association between shortening of the limbs and Down syndrome. Shortening of the humerus and of the femur to less than 0.91% of the expected length may be an indicator.
Heart defects and intestinal blockages:
These are more serious or "hard" indicators of the fetus having Down's syndrome that can also be seen on an ultrasound. There could be duodenal atresia, a condition in which the passage from the stomach to the small intestine hasn’t opened and cardiac anomalies, conditions in which the heart or vessels around it have developed abnormally.
Another indicator can be a hyper echogenic bowel, a condition in which the area around the pelvis appears brighter than normal on the ultrasound, perhaps because of a mass or an infection.
Echogenic intraventricular focus (EIF), a bright spot on the baby’s heart, which could come from calcium deposits and pleural effusion, that is, excess fluid in the chest cavity, might be detected.
Other "soft" or not so serious signs which may be associated could include-
1. Choroid plexus cysts (CPC) or a fluid-filled space in the brain.
2. “Sandal gap” toes — a larger-than-usual gap between the first and second toes
3. Clinodactyly — a pinky finger that curves slightly toward the ring finger.
These markers are not problems in and of themselves: they’ve just been shown to turn up more often in fetuses with Down syndrome. But false positives are extremely common, and markers will be found in a whopping 11 percent babies who don’t have Down syndrome.
Ultrasound after all is only a screening test. A screening test can never give you a firm diagnosis—it simply indicates that there is a higher risk of the baby having Down's syndrome.
On the other hand, a diagnostic test gives you a firm diagnosis. Only an amniocentesis and CVS are considered diagnostic tests as they directly test the genetic material of the developing baby.
A careful combination of accurately performed ultra-sonography and maternal blood testing, eventually followed by a quantitative fluorescent polymerase chain reaction (QF-PCR), should reduce the need for conventional chromosomal analysis, through amniocentesis and CVS sampling which is relatively time consuming as well as carries the risk of miscarriage.
If diagnostic tests yield positive results and parents decide to continue the pregnancy, fetal echocardiography should be performed at 20 weeks' gestation to detect serious cardiac malformations. Ultrasonography should be performed at 28-32 weeks' gestation.