First Trimester Increased Nuchal Thickness in the Fetus
by Lorraine Suslak, MS, May 1999

Fetal nuchal translucency measurement between 10 and 14 weeks gestation is recognized as a useful screening tool for fetal chromosomal abnormalities. Extensive studies have demonstrated that the combination of maternal age, nuchal translucency thickness (NTT) and the maternal serum markers, human chorionic gonadotropin (HCG) and pregnancy associated protein A (PAP A), provides an effective screening method for trisomy 21 and trisomy 18 in the first trimester of pregnancy. In some studies, the combination of first trimester analytes, maternal age and nuchal translucency thickness have yielded a detection rate of 87% for Down syndrome with a false positive rate of only 5%.

When NTT is increased and chromosome tests are normal, the question arises as to whether there is an increased chance for other fetal anomalies. Recent investigations with impressively large numbers of patients have shown a positive correlation between increased NTT and unfavorable pregnancy outcome including fetal abnormality and/or pregnancy loss.

Besides miscarriage and perinatal death, conditions most commonly found to be associated with increased NTT include disorders which have lymphatic obstruction or edema as part of their pathophysiology (e.g., left-sided cadiac outflow defects, diaphragmatic hernias, omphalocele, renal disorders, body stalk anomalies [major abdominal wall defects, severe kyphosis and rudimentary umbilical cord] as well as fetal akinesia deformation sequence). Other abnormalities may also be associated with increased NTT, however, the numbers of affected fetuses identified thus far are too small for conclusions to be drawn.

Nuchal translucency normally increases with crown-rump length. The 95th percentile for NTT for a crown-rump length of 84 mm (average for a fetal gestational age of 14 weeks 3 days) is 2.8 mm. In one study with more than 4,000 chromosomally normal singletons with an increased NTT, 96% of those with a NTT <3.4 mm resulted in healthy babies. For fetuses with a NTT of 3.5- 4.4 mm the healthy livebirth figure dropped to 92%. For a NTT of 4.5-5.4 mm, 85% resulted in healthy liveborns. Between 5.5 and 6.4 mm the figure was 65% and with a NTT of 6.5 mm or greater only 45% of newborns were healthy.

In the future it is possible that measurement of NTT between 10 and 14 weeks gestation will become a useful screening tool for all pregnancies in an effort to detect both chromosomal and non-chromosomal anomalies. Additional research, education and professional training in the technical aspects of NTT measurement are needed before widespread clinical implementation of this diagnostic method is possible.