Your Ultrasound

During the 15th and 24th week of pregnancy, or even sooner as technology improves, a diagnostic ultrasound may be performed.

Trained sonographers in a medical setting may be able to determine if a fetus is more likely to be born with Down syndrome by observing the thickness or folds of the neck. This is called nuchal translucency thickness.

Some hospitals are beginning to offer “Sequential Screening,” a two-stage test in the first trimester designed to test for Down syndrome, trisomy 18 and other chromosome disorders as well as neural tube defects such as spina bifida without the risk and invasiveness of amniocentesis or chorionic villus sampling (CVS). Participating hospitals may recommend this diagnostic ultrasound to pregnant women over age 35, or to any woman who would like to have the tests. A blood test for hormone levels in the 10th to 14th week is followed up by an ultrasound in which the amount of fluid at the back of the fetus’ neck is observed. The results of the two tests combined reveal “odds” that the fetus is affected with one of the conditions being tested for. A second blood test may be advised between the 15th and 22nd week, at which point the combined results are considered 90-percent accurate in detecting Down syndrome or trisomy 18, according to a press release from Stamford Hospital in Connecticut.

Nuchal folds, or nuchal translucency, refers to an area of fluid at the back of the fetus’ neck. This is an area of medicine that is still being studied, as researchers examine the correlation between increased nuchal translucency and Down syndrome, as well as other issues with the fetus. More recently, the absence of a fetal nasal bone has been recognized as an indicator as well.

An increased desire to and the medical possibility of learning of fetal anomalies during the first trimester has led to great interest in using ultrasound to determine if a fetus has Down syndrome. Sometimes, a doctor will order an ultrasound as an additional possible indicator when other factors (such as a maternal serum screening) indicate an increased possibility of Down syndrome. Perhaps you’ve opted against or been advised not to have more-invasive prenatal testing such as amniocentesis or a chorionic villus sample (CVS) test, which are usually not offered to women under 35. Maybe you’re now wondering if an ultrasound will reveal the presence of Triosomy 21, or Down syndrome. The answer is, maybe—to a professional.

But even doctors, sonographers and other highly trained experts viewing a fetal ultrasound have a significant error rate when it comes to diagnosing Down syndrome. Studies conflict on the accuracy rate of Down syndrome diagnoses based solely on ultrasound, with some medical journal articles reporting an error rate as high as 50 percent. If you’re a layperson, it’s really not appropriate to think that you can assess such a condition by observing an ultrasound, especially a printout of an ultrasound image. Nuchal translucency is measured in fractions of millimeters, multiple times, using carefully calibrated calipers and advanced ultrasound technology.

Characteristics, or “ultrasonic soft markers,” that could indicate Down Syndrome can be sought around week 13 or 14, in the second trimester. Other congenital issues may not be apparent until week 18 to 20, or even later.

By the second trimester, when the fetus is more developed, a medical professional is more likely to observe the same types of differences that would be seen in a full-term baby with Down syndrome, such as a shorter nose, ears set lower on the head and/or shorter arm and leg bone as well as differences within the internal organs.

In summary, prenatal diagnosis of Down syndrome becomes more accurate as the variety of tests increase and the pregnancy progresses. It’s not advisable for a layperson to rely solely on an ultrasound when gauging whether a fetus has Down syndrome.

This type of reading is best left to a well-trained sonographer or technician in consult with a physician. Learning that a fetus may have Down Syndrome can be an emotional experience, and an attentive physician will anticipate any concerns the patient may have.