Role of ultrasound evaluation of nuchal translucency in prenatal diagnosis.

BACKGROUND: Nuchal translucency (NT) is the ultrasonographic pattern of the accumulation of subcutaneous fluid (> or = 3 mm) behind the fetal neck. The measurement of NT thickness by ultrasound examination at 11-14 weeks of gestation has been associated with maternal age and to be an effective screening tool for trisomy 21; with an invasive method rate of 5%, about 75% of trisomical pregnancies can be identified. With the association of some biochemical markers like maternal serum free beta-human chorionic gonadotropin (beta-hCG) and pregnancy-associated plasma protein-A (PAPP-A) to ultrasonography at 11-14 weeks, it is possible to identify about 90% of chromosomal abnormalities. An increase of NT also allows us to identify most other chromosomal abnormalities, a large number of major cardiac defects, skeletal dysplasias, and genetic syndromes. In monochorial twins the discordance in the measurement of NT represents an early sign of twin-to-twin transfusion syndrome (TTTS).

METHODS: The objective of our study was to assess the detection of fetal structural defects with an ultrasound scan at 11-14 weeks of gestation. We submitted 3,157 pregnant women to a routine ultrasound examination at 11-14 weeks. The patients were then submitted to ultrasound scan in the second or third trimester of pregnancy. An isolated increased NT thickness was not considered an abnormality, but these patients, nonetheless, were submitted to an early echocardiographic evaluation. Fetal structural abnormalities were classified as major or minor and of early or late onset.

RESULTS: A detection rate of 4.3% (135 cases) of abnormalities was found and 22.6% of these (30 cases) were diagnosed by ultrasound scan at 11-14 weeks, including seven cardiac defects associated with increased NT. The antenatal ultrasound detection rate was 73.5%, and 33.2% were diagnosed in the first trimester assessment. A rate of 76.8% of the major defects were diagnosed by the prenatal scan and 35.8% by the scan at 11-14 weeks. Fetal structural defects with the ultrasonography at 11-14 weeks were diagnosed in about 24.3% of the cases, therefore, a second trimester abnormality is important in routine antenatal care to increase the prenatal assessment of fetal anomalies.

CONCLUSIONS: As for the introduction of every new technique in routine clinical practice, the operators who perform the ultrasound scan at 11-14 weeks should be submitted to adequate training and to strict quality control.