Measurement of fetal nasal width by ultrasonography

Second trimester ultrasound prenasal thickness combined with nasal bone length: a new method of Down syndrome screening

OBJECTIVE

To evaluate ultrasound prenasal thickness (PT) as a way of enhancing the Down syndrome (DS) screening performance of second-trimester nasal bone (NB) length measurement.

METHODS

Twenty-one DS and 500 normal fetuses were scanned at 14-27 weeks' gestation. The affected karyotype was known to the person performing the scan. Both PT and NB were measured in a mid-sagittal position and results were expressed in multiples of the normal gestation-specific median (MoM). Gaussian modelling was used to predict the performance of routine screening.

RESULTS

Among DS fetuses, the PT median was 1.35 MoM, a highly statistically significant increase compared with the unaffected pregnancies (p < 0.0001, Wilcoxon Rank Sum Test, two-tailed). The NB median was 0.87 MoM (p < 0.0005) and the ratio PT/NB median was 1.51 MoM (p < 0.0001). In the normal fetuses, the two markers were positively correlated (0.25, p < 0.0001), whereas in the DS fetuses there was a non-significant negative correlation (-0.24, p = 0.33). For a 5% false-positive rate, the model predicted detection rate was 70% for PT and NB compared with 43% for NB alone.

CONCLUSION

In the second trimester, PT is increased on average in DS fetuses. Combining PT and NB measurement could yield a higher screening detection rate than NB alone. Confirmation of our findings in a series of women scanned before the karyotype was known is needed before the method can be clinically implemented.

Prenatal diagnosis of Freeman–Sheldon syndrome and usefulness of an ultrasound fetal lip width normogram

OBJECTIVES

Freeman-Sheldon syndrome (FSS) is a rare genetic condition with facial and joint abnormalities. Here we report the first case of prenatal diagnosis in a fetus with negative familial history for this syndrome. A chart of the fetal mouth length was generated and its usefulness verified.

METHODS

A cross-sectional study was carried out to derive standard charts for fetal lip lengths. The study population was made up of 371 fetuses between 15 and 38 weeks of gestation examined only once for this purpose. A statistical analysis previously recommended to derive normal ranges in fetal growth charts was employed.

RESULTS

A 34-year-old woman was referred at 28 weeks of gestation for polyhydramnios and clubfeet. A detailed scan confirmed those findings and showed joint contractures of hands, a severe microstomia with a 'whistling' mouth, and an H-shaped mark over the chin. FSS was suspected, though there was no family history for this genetic syndrome. After emergency delivery at 32 weeks, the syndrome was confirmed. The infant died two months later from pneumonia. Upon construction of a standard mouth length chart, it was found that the fetal lip width in this case of FSS was well below the 5th centile.

CONCLUSION

Here we present a standard chart for fetal lip width. We demonstrated its usefulness in cases with fetal face anomalies. Specific charts are fundamental for an objective assessment of the fetal parameters.

Use of genetic sonography for adjusting the risk for fetal Down syndrome

Systematic evaluation of ultrasound findings known to be associated with trisomy 21, at an appropriate gestational age, has been referred to as a genetic sonogram. A number of high-risk centers performing genetic sonography have reported detection of ultrasound abnormalities in the majority of fetuses with fetal Down syndrome. However, nonspecific markers are more commonly observed than structural abnormalities, which are detected in less than 20% of cases in a nonselected population. Also, the actual sensitivity of a genetic sonogram will depend on various factors including the markers sought, gestational age, reasons for referral, and of course the quality of the ultrasound. Appropriate use of a genetic sonogram can help to modify the risk of fetal Down syndrome by decreasing the risk when the ultrasound is normal, or increasing the risk when specific ultrasound markers are detected. The postultrasound risk can be estimated by applying specific likelihood ratios, reflecting the strength of individual markers, with the a priori risk based on maternal age alone, or combined with biochemical markers when known. We review this approach of age-adjusted ultrasound risk assessment for fetal Down syndrome and illustrate how the risk can be estimated. Individual sonographic markers are also discussed.

Fetal nasal bone length: reference range and clinical application in ultrasound screening for trisomy 21

OBJECTIVES

Fetuses with trisomy 21 typically present with subtle facial abnormalities, including a hypoplastic nasal bone. The aim of this study was to provide a reference range for the length of the fetal nasal bone and to test its value in second-trimester ultrasound screening for trisomy 21.

DESIGN

A reference range of fetal nasal bone length was established from cross-sectional data on 1923 consecutive singleton pregnancies scanned at 16-24 weeks' gestation in women older than 35 years. Screening for trisomy 21 was prospectively studied using the measurement of fetal nasal bone lengths smaller than the 5(th) percentile as a cut-off value.

RESULTS

Follow-up was possible in 1631 cases (84.8%). Trisomy 21 was found in 22 cases (1.35%). Nasal bone length measurement increased as a function of gestational age (P < 0.05) showing a linear relationship. Screening for trisomy 21 using the 5(th) percentile as a cut-off value resulted in a sensitivity of 59.1% for a 5.1% false-positive rate. The likelihood ratio was 11.6.

CONCLUSION

Screening for trisomy 21 using fetal nasal bone length measurements showed a sensitivity comparable to that of maternal biochemistry for a given false-positive rate of 5%. Association of nasal bone lengths with other sonographic markers, taking into account the background risk for maternal and gestational age, may further improve sensitivity and reduce false positives, allowing avoidance of unnecessary invasive diagnostic procedures.

Craniofacial and neck anomalies

The evaluation of the craniofacial region is a multistep process. First, sonographic skill and expertise are required to ascertain these often subtle abnormalities. Next, precise measurements must be obtained in appropriate and reproducible planes. Finally, a thorough search for other related fetal anomalies is essential. Exciting breakthroughs in our understanding of underlying courses and mechanisms can now guide the practitioner not only in deciding whether or not to pursue invasive testing, but which tests to order, particularly if molecular diagnosis may be required. Complementary advances in ultrasound technology, prenatal diagnosis, and genetic research will have the potential to enhance the accuracy of our counseling, management, and overall care of our patients.