The mid-trimester genetic sonogram

Contribution of "modified genetic sonography" to the combined test as a screening method for chromosomal abnormalities

OBJECTIVES

We propose to assess the contribution of "modified genetic sonography" (MGS) to the combined test (CT) as a method of stepwise sequential screening (1st step: CT, 2nd step: MGS) for chromosome abnormalities in the general population of pregnant women.

METHODS

Prospective study. During a 4 year study period (July 2005-June 2009) 16,548 pregnancies underwent a CT combined with MGS (major malformation and nuchal fold) as a screening method for chromosome abnormalities. We assessed sensitivity and false positive rate (FPR) (95% CI).

RESULTS

We offered a chromosome abnormalities screening test to 96.6% of pregnancies (15,995 cases). 14,160 cases are analyzed (1st step: CT, 2nd step: MGS) including 49 chromosome abnormalities and 35 Down's syndrome (DS). The sensitivity of CT for DS was 77.1% [95% CI, 63.2-91] (27/35) and 77.5% for all chromosome abnormalities [95% CI, 65.8-89.2] (38/49) with a FPR of 4.4% [95% CI, 4.1-4.7]. If MGS was combined with CT, the sensitivity for DS was 91.4% [95% CI, 82.1-99] (32/35) and 93.8% for all chromosome abnormalities [95% CI, 87-99] (46/49) for a FPR of 5.1% [95% CI, 4.7-5.5].

CONCLUSIONS

The addition of an MGS to combined first-trimester screening test for aneuploidy improved sensitivity by 14.3% while only increasing the FPR by 0.7%.

Second-trimester genetic sonogram for detection of fetal chromosomal abnormalities in a community-based antenatal testing unit

OBJECTIVE

To evaluate the efficacy of the second-trimester genetic sonogram for the detection of Down syndrome and other chromosomal abnormalities in a community-based antenatal testing unit.

METHODS

This was a retrospective study using data from two community hospital antenatal ultrasound units. Six hundred and sixty fetal ultrasound examinations in both at-risk (n = 581) and low-risk (n = 79) pregnancies were performed from 15 + 0 to 22 + 6 weeks' gestation and all cases were verified for outcome data. The sonographic detection of a major congenital anomaly or a sonographic marker (increased nuchal skinfold, short humerus, short femur, echogenic bowel, pyelectasis, echogenic intracardiac focus, absence or hypoplasia of fifth mid phalanx or choroid plexus cyst) was recorded. The entire group of 660 ultrasound examinations as well as subgroups with and without non-ultrasound risk factors for a fetal chromosomal abnormality were analyzed to determine the sensitivity, specificity, positive and negative predictive values and positive likelihood ratio for the detection of Down syndrome and other fetal chromosomal abnormalities.

RESULTS

There were 32 (4.85%) chromosomal abnormalities in our study population. Twelve (3.75%) of these were Down syndrome, of which eight (66.6%) had a positive ultrasound examination in the second trimester. Six of seven (85.7%) of the trisomy 18 fetuses, 2/2 of the trisomy 13 fetuses and 2/3 of the non-mosaic 45,X fetuses had positive sonograms. The overall detection rate for chromosomal abnormalities was 20/32 (sensitivity, 62.5%; specificity, 80.7%; negative predictive value, 97.7%; positive predictive value, 14.2%; positive likelihood ratio, 3.24). Major structural defects and sonographic markers, excluding hypoplastic fifth digit and choroid plexus cyst, occurred significantly more frequently in Down syndrome cases compared with normal ones.

CONCLUSIONS

In a community-based antenatal testing unit we have demonstrated a detection rate for fetal Down syndrome with the second-trimester genetic sonogram that is comparable to the range of sensitivities reported by larger centers involving primarily high-risk patients. However, only 12 of the 32 fetal chromosomal abnormalities that we encountered were Down syndrome.

Comparison of the efficiency of second-trimester nasal bone hypoplasia and increased nuchal fold in Down syndrome screening

OBJECTIVE

The purpose of this study was to compare the efficiency of second-trimester nasal bone hypoplasia (NB) and increased nuchal fold (NF) in Down syndrome screening.

STUDY DESIGN

This was a prospective multicenter cohort study of women who underwent an anatomic survey between 16 and 22 weeks of gestation. The fetal NB and other markers of fetal aneuploidy that included NF were evaluated. NB was defined either as an absent NB or length of <0.75 multiples of the median for the gestational age. Two definitions of increased NF (>5 mm and >6 mm) were evaluated. Fetuses or infants with Down syndrome were compared with those fetuses without for the presence of NB and increased NF.

RESULTS

Among 4373 pregnancies that were evaluated over a 5-year period, there were 50 pregnancies with Down syndrome. NB evaluation was obtained in 3936 of 4373 pregnancies (90%); NF was evaluated in all of the fetuses. Absent NB was seen in 14/49 cases (29%), and NF of >6 mm was seen in 6 of 50 cases (12%) with Down syndrome.

CONCLUSION

Nasal bone and nuchal fold are efficient markers for Down syndrome. Absent nasal bone was a better predictor of Down syndrome, compared with nuchal fold, and should be a standard marker when a second-trimester genetic sonogram is performed.

Expected performance of second trimester maternal serum testing followed by a 'genetic sonogram' in screening of fetal Down syndrome

OBJECTIVE

To assess the efficacy of screening for fetal Down syndrome through the sequential provision of second trimester maternal serum tests and fetal ultrasound (the 'genetic sonogram').

METHODS

Monte Carlo modeling was used to generate typical results for second trimester serum screening. Serum test likelihood ratios were then multiplied by likelihood ratios associated with the provision of a genetic sonogram. The impact of adding the genetic sonogram on the detection rate (DR) and false-positive rate (FPR) were assessed for typical protocols that are currently in use. The effect of expanding the number of women who receive the genetic sonogram, variation in the marker frequencies in the genetic sonogram, and a multivariate Gaussian model that incorporated both serum and ultrasound measurements as continuous variables were also considered.

RESULTS

When the genetic sonogram is offered only to those women who are screen-positive by serum testing, there can be a substantial reduction in the number of women with an indication for amniocentesis but also some loss in detection. The extent of these changes will partially depend on the serum tests and cutoff used. Providing the genetic sonogram to more women can reduce loss in detection without resulting in high amniocentesis rates. As a sequential screening tool, the genetic sonogram can be improved by incorporating markers that have a high frequency in affected pregnancies and by using ultrasound measurements as continuous variables.

CONCLUSION

It should be possible to provide highly effective multistep screening protocols that maximize the benefits of both maternal serum and ultrasound while minimizing the amount of testing offered.

Second trimester prenatal ultrasound for the detection of pregnancies at increased risk of Down syndrome

OBJECTIVE

To determine the association between second trimester ultrasound findings (genetic sonogram) and the risk of Down syndrome.

METHODS

Prospective population-based cohort study of women who were at increased risk of chromosome abnormality based on serum screening.

RESULTS

Overall 9244 women with singleton pregnancies were included, including 245 whose fetuses had Down syndrome. Overall, 15.3% of the women had an abnormal genetic sonogram, including 14.2% of pregnancies with normal fetuses and 53.1% of those with Down syndrome. If the genetic sonogram were normal, the risk that a woman had a fetus with Down syndrome was reduced (likelihood ratio 0.55 [95% CI 0.49, 0.62]) However, if the normal genetic sonogram were used to counsel these high-risk women that they could avoid amniocentesis, approximately half of the cases of Down syndrome (115 of 245) would have been missed. The isolated ultrasound soft markers were the most commonly observed abnormality. These were seen in a high proportion of Down syndrome fetuses (13.9%) and normal fetuses (9.3%). In the absence of a structural anomaly, the isolated ultrasound soft markers of choroid plexus cyst, echogenic bowel, renal pyelectasis, clenched hands, clinodactyly, two-vessel umbilical cord, short femur, and short humerus were not associated with Down syndrome. Nuchal fold thickening was a notable exception, as a thick nuchal fold raised the risk of Down syndrome even when it was seen without an associated structural anomaly.

LIMITATIONS

All women included in this study were at high risk of Down syndrome based on serum screening, and thus the results of this study cannot be used as a basis to modify maternal age-related risk.

CONCLUSIONS

The accuracy of the genetic sonogram is less than previously reported. The genetic sonogram should not be used as a sequential test following serum biochemistry, as this would substantially reduce the prenatal diagnosis of Down syndrome cases. In contrast to prior reports, most isolated soft markers were not associated with Down syndrome.

Evaluating the efficiency of using second-trimester nasal bone hypoplasia as a single or a combined marker for fetal aneuploidy

OBJECTIVE

Although second-trimester nasal bone (NB) hypoplasia has been associated with fetal aneuploidy, its role as a single marker is still uncertain. Our objective was to evaluate the efficiency of NB hypoplasia as an independent marker for fetal aneuploidy.

METHODS

This was a prospective cohort study of women undergoing an anatomic survey between 16 and 22 weeks' gestation. The fetal NB and other markers of fetal aneuploidy, including nuchal fold, femur and humeral lengths, choroid plexus cysts, major fetal anomalies, echogenic bowel, pyelectasis, and hypoplastic fifth digits, were evaluated. Nasal bone hypoplasia was defined either as an absent NB or by a ratio of the biparietal diameter to NB. Fetuses or infants with fetal aneuploidy were compared with those without for the presence of NB hypoplasia either as a single marker or in the presence of other markers for aneuploidy.

RESULTS

Of 2885 women evaluated, NB measurements were obtained in 2465 (85%). There were 35 (1.4%) cases with fetal aneuploidy. The sensitivity and specificity of a single NB in detecting Down syndrome varied from 23% to 64% and 57% to 99%, respectively, depending on the definition of NB hypoplasia used. There was an improvement in the efficiency of using the NB when combined with other markers, with sensitivity and specificity increasing from 59% to 82% and 74% to 87%, respectively.

CONCLUSIONS

Nasal bone hypoplasia is a marker for fetal aneuploidy. The combination of the NB with other makers was associated with an improvement in detection of fetal aneuploidy.