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

Authors

  • J. N. Bottalico,

    Corresponding author
    1. University of Medicine and Dentistry of New Jersey, School of Osteopathic Medicine and Kennedy Memorial Hospitals, Stratford and Turnersville, NJ, USA
    • UMDNJ-SOM Department of Obstetrics and Gynecology, UDP Suite 3600, Stratford, New Jersey, 08084, USA
    Search for more papers by this author
  • X. Chen,

    1. University of Medicine and Dentistry of New Jersey, School of Osteopathic Medicine and Kennedy Memorial Hospitals, Stratford and Turnersville, NJ, USA
    Search for more papers by this author
  • M. Tartaglia,

    1. University of Medicine and Dentistry of New Jersey, School of Osteopathic Medicine and Kennedy Memorial Hospitals, Stratford and Turnersville, NJ, USA
    Search for more papers by this author
  • B. Rosario,

    1. University of Medicine and Dentistry of New Jersey, School of Osteopathic Medicine and Kennedy Memorial Hospitals, Stratford and Turnersville, NJ, USA
    Search for more papers by this author
  • D. Yarabothu,

    1. University of Medicine and Dentistry of New Jersey, School of Osteopathic Medicine and Kennedy Memorial Hospitals, Stratford and Turnersville, NJ, USA
    Search for more papers by this author
  • L. Nelson

    1. University of Medicine and Dentistry of New Jersey, School of Osteopathic Medicine and Kennedy Memorial Hospitals, Stratford and Turnersville, NJ, USA
    Search for more papers by this author

Abstract

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. Copyright © 2009 ISUOG. Published by John Wiley & Sons, Ltd.

Introduction

Although first-trimester and first- and second-trimester sequential sonographic and biochemical screening for fetal Down syndrome and trisomy 18 are being utilized increasingly, second-trimester sonographic risk assessment remains an important component of obstetric practice, with a role in the detection of pregnancies affected by Down syndrome, trisomy 18 and open neural tube defects.

Until recently, peer-reviewed literature addressing the sensitivity of second-trimester ultrasound for the detection of fetal chromosomal abnormalities focused largely on Down syndrome in pregnancies with increased risk and reflected the experience of large institutions with high-volume services. A multicenter study of the efficacy of the second-trimester genetic sonogram in the detection of Down syndrome in high-risk pregnancies reported a combined sensitivity of 71.6%1. Detection rates for Down syndrome in sonographically screened low-risk populations have become available only recently2, 3 and controversy exists regarding the significance of sonographic markers, especially when isolated, for the detection of Down syndrome as well as the role of genetic sonography for further risk modification after first- or second-trimester combined or biochemical screening4, 5. For community hospital-based maternal–fetal medicine (MFM) and other specialists it can be difficult to counsel women accurately about their individual risk after a second-trimester ultrasound examination without knowledge of the center-specific detection and false-positive rates. Extrapolation of published information from referral centers should be done with caution considering that even in ‘specialized’ centers there is a wide range of sensitivities1–4, 6, 7. There have been few studies reporting the efficacy of community-based second-trimester ultrasound screening for the detection of chromosomally abnormal fetuses. One such study, by Wax et al.8, on the efficacy of community-based genetic sonography in detecting chromosomally abnormal fetuses, reported that 82% of aneuploid fetuses had an abnormal sonogram (one or more markers present), including 5/7 (71.4%) with Down syndrome, in an exclusively high-risk population. We therefore sought to report the screening efficacy of second-trimester genetic sonography for Down syndrome as well as other significant chromosomal abnormalities in our community-based prenatal diagnosis program.

Methods

This was a retrospective review of second-trimester ultrasound data from the computerized databases of two community hospital-based antenatal testing units that included eligible studies performed between February 2000 and November 2003. The study group was recruited from two suburban hospitals in the US Mid-Atlantic region and included 660 second-trimester fetal ultrasound examinations performed in the two hospitals, which have a combined total of approximately 1800 deliveries per year. Ultrasound examinations were performed by obstetric Registered Diagnostic Medical Sonographers (RDMS) supervised by two MFM specialists. Most fetuses were scanned by both a sonographer and a physician. Genetic counseling, when indicated, was performed by a board-certified genetic counselor.

Cases were included in the analysis if they were performed between 15 + 0 weeks and 22 + 6 weeks of gestation, with the following indications: maternal age ≥ 35 years at term (advanced maternal age, AMA), positive second-trimester maternal serum screen for a chromosomal anomaly, the detection in our facility of a major congenital anomaly or sonographic markers of fetal aneuploidy, or a positive family history of a previous pregnancy affected by a chromosomal abnormality.

Maternal serum screening was performed by the ‘triple screen’ using alpha-fetoprotein, human chorionic gonadotropin and unconjugated estriol, along with maternal age to establish second-trimester risk estimates of Down syndrome. The screen was considered positive when risk estimates were ≥ 1/270. Ultrasound examinations were considered positive if either a major congenital anomaly or one or more sonographic markers were detected. Major malformations were defined as fetal structural anomalies that would either require surgery after birth or cause major morbidity and/or mortality. Sonographic markers were defined as follows. Nuchal skinfold (NSF) thickening was defined as ≥ 6 mm, measured according to standard methods and previously published criteria9–11. Choroid plexus cysts, either single or multiple, were considered present and recorded as positive if they were ≥ 3 mm in diameter. Mottling of the choroid plexi was not considered significant. Short femur and short humerus were defined according to previously published regression formulae that use the biparietal diameter in the calculation of expected long-bone lengths. An observed-to-expected femur length ≤ 0.91 was considered a positive marker12 (expected femur length = − 9.3105 + 0.9028 × BPD) and an observed-to-expected humerus length < 0.90 was considered a positive marker13 (expected humerus length = − 7.9404 + 0.8492 × BPD). The fetal bowel was considered echogenic if the degree of echogenicity was equal to or greater than that of bone, typically using the adjacent iliac wing for comparison. Enhanced echogenicity was not considered significant if it was seen only with high-frequency transducers that caused overall enhancement of image echogenicity. Echogenic intracardiac foci, either single or multiple, were considered positive if they were present in the fetal left or right ventricle, most typically in the chordae tendineae and/or papillary muscles. Absence or hypoplasia of the fifth middle phalanx was recorded as either positive or negative and was not defined rigorously based on measurements, but was determined by the attending MFM physician. Renal pyelectasis was recorded positive if the anteroposterior diameter of one or both renal pelves, measured in a transverse section of the kidneys, was ≥ 4 mm. Appearance and size of the nasal bone were not evaluated consistently at the two centers during the time period of this study, so nasal bone data were not included in the analysis. Family histories were recorded as being positive if there was a known chromosomal abnormality in either parent or in first-degree relatives.

We reviewed cytogenetic reports from all genetic amniocenteses performed during the study period. Pregnancy terminations were reviewed for the availability of cytogenetic results from fetal or placental tissue. Maternal hospital records were cross-checked for pregnancy outcomes. Neonatal records were searched based on ICD-9 codes14 for chromosomal abnormalities in general (758.x) and Down syndrome specifically (758.0). All neonates were examined by the attending pediatrician or neonatologist at birth. Neonates with normal phenotypes were assumed to have normal karyotypes. Cases were excluded if neonatal outcome could not be verified or if fetal karyotype was not available from a pregnancy termination.

Data were recorded as categorical variables (positive or negative) for: AMA (maternal age ≥ 35 years at term); positive maternal serum screen; family history of a chromosomal abnormality; presence of a major malformation, choroid plexus cysts, (either single or multiple), short femur, short humerus, increased NSF, echogenic bowel, echogenic intracardiac focus, absence or hypoplasia of fifth middle phalanx or renal pyelectasis. Data were entered into a computer database and imported for statistical analysis using SAS v.9.1 (SAS Institute, Inc., Cary, NC, USA) for calculations of sensitivity, specificity, positive (PPV) and negative (NPV) predictive values and positive likelihood ratios (+LR) by group as defined below. Logistic regression analysis was performed to predict Down syndrome or any major chromosomal abnormality from various ultrasound findings using chromosomal abnormalities as the dependent variables and ultrasound abnormalities as the independent variables. Odds ratios (OR) and 95% CIs from the logistic regression coefficients and their corresponding covariance matrices were computed for the various sonographic findings with respect to Down syndrome detection. + LRs were calculated using the equation + LR = Sensitivity/1 − Specificity.

Cases were stratified into the following groups: Group 1, AMA only; Group 2, positive serum screen (any maternal age) or AMA plus a positive family history of a chromosomal abnormality; Group 3, not AMA, normal or unknown maternal serum screen, positive or negative family history, and positive genetic sonogram. In Group 1 (n = 457), the a priori risk of a fetal chromosomal abnormality was driven largely by maternal age alone since only 3.5% of women in this group had a triple screen test and it had to be negative for group assignment. In Group 2 (n = 124), a positive triple screen defined the a priori risk (Down syndrome risk ≥ 1:270) for the majority. In Group 3 (n = 79), the a priori risk was low (Down syndrome risk < 1:270) based on the negative, declined or unknown triple screen. Thus there was a range of a priori risks across the three groups. The sensitivity, specificity, PPV, NPV and + LR for nine ultrasound variables were calculated for each of the three groups with regard to trisomy 21 only, either trisomy 18 or 13, and any/all chromosomal abnormalities.

Groups 1 and 2 were combined to allow clinically meaningful test performance calculations since both comprised the at-risk group. Ultrasound test performance parameters were also calculated for the entire study population with regard to the occurrence of all chromosomal abnormalities as well as subgroups of chromosomal abnormalities. The number of chromosomal abnormalities was compared in those who underwent genetic amniocentesis with those who did not and with regard to the types of maternal risk factors present and whether ultrasound examinations were positive or negative.

The study was approved by the joint institutional review board of the University of Medicine and Dentistry of New Jersey School of Osteopathic Medicine and Kennedy Health System in Stratford New Jersey.

Results

A total of 660 ultrasound examinations were eligible for inclusion in the study, 457 in Group 1, 124 in Group 2 and 79 in Group 3. Genetic amniocenteses were performed in 350 (53%) cases, with 26 (7.4%) cases having a cytogenetic abnormality. In the 310 cases that declined genetic amniocentesis, there were six (1.9%) cytogenetic abnormalities, four diagnosed postnatally by peripheral blood karyotyping after detection of an abnormal phenotype (three trisomy 21, one trisomy 18), one diagnosed by third-trimester cordocentesis in a hydropic fetus with trisomy 21 and one diagnosed after mid-trimester termination for major congenital anomalies in a 45,X fetus. A total of 32 chromosomal abnormalities were eventually found in the total study population: 12 (37.5%) cases of trisomy 21; seven (21.9%) of trisomy 18; four (12.5%) of 45,X (three non-mosaic, one mosaic 45,X/46,XX); two (6.3%) of trisomy 13; one (3.1%) of 69,XXY and six (18.7%) of miscellaneous cytogenetic abnormalities.

Table 1 gives the frequencies of the various sonographic findings in all 660 examinations, with NSF data categorized by gestational age. Most examinations (80%) were performed in the 15th to 19th weeks of gestation and of the eight cases with increased NSF, only two (which had exams at 16 and 19 weeks) had trisomy 21.

Table 1. Summary of frequency of sonographic findings (n = 660)
Findingn (%)
  1. GA, gestational age; NSF, nuchal skinfold.

Choroid plexus cyst55 (8.3)
Echogenic intracardiac focus35 (5.3)
Echogenic bowel14 (2.1)
Major congenital anomaly37 (5.6)
Absent/hypoplastic fifth mid phalanx2 (0.3)
Increased NSF8 (1.2)
 At 15 + 0 to 19 + 6 weeks5
 At 20 + 0 to 22 + 6 weeks3
Pyelectasis13 (2.0)
Short femur9 (1.4)
Short humerus10 (1.5)
GA at ultrasound 
 15 + 0 to 19 + 6 weeks531 (80.5)
 20 + 0 to 22 + 6 weeks129 (19.6)

Table 2 lists the numbers and specific types of fetal chromosomal abnormalities with reference to the ultrasound findings and whether AMA was a factor. Of the 12 Down syndrome fetuses, eight (66.7%) had positive second-trimester ultrasound examinations, 4/7 (57.1%) in the AMA group, and 4/5 (80.0%) in the < 35 years age group. In the three Down syndrome fetuses in women ≥ 35 years who had negative second-trimester ultrasound examinations, one fetus had no demonstrable ultrasound findings until the third trimester, so this case was considered a false negative in the second trimester. The other two both had negative second-trimester sonograms; one was diagnosed by genetic amniocentesis and one was diagnosed in the neonatal period; thus these also were considered as false-negative second-trimester ultrasound studies. The one Down syndrome case with a normal genetic sonogram in a mother aged < 35 years involved a missed congenital cardiac anomaly (an atrioventricular septal defect). Six of seven (85%) of the trisomy 18 fetuses had abnormal second-trimester sonograms and in all six positive examinations, major malformations were detected. In the one fetus with trisomy 18 detected only by genetic amniocentesis at 16 weeks, no major congenital anomalies or ultrasound markers were demonstrable on ultrasound but fetal cardiac and hand views were suboptimal. All pregnancies affected with fetal trisomy 13 and triploidy and one case with interstitial deletion of 5q in a twin had major malformations detected by genetic sonogram. Two of three cases with non-mosaic Turner syndrome (45,X) had major congenital anomalies detected on ultrasound. Genetic sonograms were normal in cases involving mosaic Turner syndrome (45,X/46,XX), a reciprocal translocation t(2 : 7), a pericentric inversion in chromosome 5, one marker chromosome and in two cases with derivative chromosomes. Of the four pregnancies with fetal Turner syndrome, three (two 45,X and one 45,X mosaic) had abnormal serum screens, all indicating an increased risk of Down syndrome. Wenstrom et al.15 previously reported that the triple screen will detect Turner syndrome as well as trisomies 21 and 18. Overall, in the entire group of 32 with chromosomal abnormalities, 20 (62.5%) had an abnormal ultrasound examination in the second trimester.

Table 2. Karyotype and genetic sonogram results of 32 fetuses with chromosomal abnormalities
Abnormal fetal karyotypeCases (n)MA ≥ 35 yearsMA < 35 yearsTotal with abnormal US (n (%))
nWith abnormal US (n (%))nWith abnormal US (n (%))
  • *

    Sensitivity, 62.5%; specificity, 80.7%; negative predictive value, 97.7%; positive predictive value, 14.2%; positive likelihood ratio, 3.24. N/A, not applicable; US, second-trimester ultrasound examination.

Trisomy 211274/7 (57)54/5 (80)8/12 (66)
Trisomy 18754/5 (80)22/2 (100)6/7 (85)
Trisomy 1320N/A22/2 (100)2/2 (100)
45,X310/1 (0)22/2 (100)2/3 (66.6)
Inversion110/1 (0)0N/A0
Translocation110/1 (0)0N/A0
Marker chromosome110/1 (0)0N/A0
Triploidy10N/A11/1 (100)1/1 (100)
Derivative chromosomes20N/A20/20
Deletion10N/A11/1 (100)1/1 (100)
45,X mosaic10N/A100
Total32168/16 (50)1612/16 (75)20/32 (62.5*)

Table 3 gives odds ratios and 95% CIs specific for Down syndrome for eight isolated markers and major congenital anomalies as well as for the presence of multiple markers when more than one non-anomaly marker was present. The wide 95% CI for increased NSF was likely due to the small number of cases (n = 8) with NSF thickening; two of these fetuses had Down syndrome.

Table 3. Odds ratios (OR) and 95% CIs for detection of Down syndrome by sonographic markers
MarkerOR95% CI
  • *

    Not significant.

  • Model fit was not valid for calculation of OR and 95% CI. NSF, nuchal skinfold.

Increased NSF20.733.72, 115.55
Major congenital anomaly9.182.32, 26.27
Short femur17.743.27, 96.26
Short humerus15.502.92, 82.40
Echogenic bowel12.362.34, 34.96
Echogenic intracardiac focus9.972.84, 34.96
Pyelectasis22.935.31, 99.02
Multiple sonographic markers13.133.87, 44.59
Choroid plexus cyst*1.010.13, 7.95
Absent/hypoplastic fifth mid phalanx

Table 4 contains calculations for the at-risk subgroups (Groups 1 and 2; with AMA, positive triple screen at any age or AMA plus a positive family history, i.e. pregnancies with an elevated background risk of fetal chromosomal anomaly) analyzed specifically for the detection of Down syndrome. After logisitic regression analyses, calculation of odds ratios and 95% CIs for the various sonographic findings revealed no significant association between the presence of choroid plexus cysts or fifth mid phalanx findings, these were eliminated as Down syndrome markers. For the sonographic detection of Down syndrome, the highest PPV and + LR (66.7% and 82.73, respectively) were associated with fetal NSF thickening at 15 + 0 to 19 + 6 weeks (sensitivity, 18.2%; specificity, 99.8%; negative predictive value, 98.1%).

Table 4. Efficacy of second-trimester genetic sonogram for detection of Down syndrome in 567 at-risk pregnancies (Groups 1 and 2; including 11 Down syndrome cases)
Ultrasound findingsSensitivity (%)Specificity (%)Negative predictive value (%)Positive predictive value (%)Positive likelihood ratio*
  • Group 1 = advanced maternal age (AMA)≥ 35 years at expected date of delivery and no other risk factor; Group 2 = positive serum screen (any age; positive triple screen with second-trimester Down syndrome risk estimate of ≥ 1/270) or AMA plus positive family history of a chromosomal abnormality.

  • *

    Positive likelihood ratio = Sensitivity/1 − Specificity. NSF, nuchal skinfold.

Increased NSF 15 + 0 to 19 + 6 weeks18.1899.7898.0666.6782.73
Pyelectasis27.2798.9298.5733.3325.27
Short femur18.1899.2898.4033.3325.27
Major congenital anomaly27.2798.5698.5627.2718.95
Short humerus18.1898.9298.3925.0016.85
> 1 marker27.2798.3898.5625.0016.85
Echogenic bowel9.0999.2898.2220.0012.64
Echogenic intracardiac focus27.2797.4898.5517.6510.83
Any 1 marker36.3690.4798.637.023.81

Table 5 contains calculations for the entire study population analyzed specifically for the detection of Down syndrome. Again, the highest positive predictive value and + LR (40% and 28.06, respectively) for the detection of Down syndrome were associated with increased fetal NSF at 15 + 0 to 19 + 6 weeks (sensitivity, 16.7%; specificity, 99.4%; negative predictive value, 98.0%). The presence of more than one sonographic marker conferred much higher PPVs and + LRs than did the presence of an isolated marker, unless the latter was a major congenital anomaly. Only 1/79 pregnancies in the low-risk group with a positive genetic sonogram (Group 3) was affected with Down syndrome.

Table 5. Efficacy of second-trimester genetic sonogram for detection of Down syndrome in total study population (Groups 1–3; n = 640*; including 12 Down syndrome cases)
Ultrasound findingsSensitivity (%)Specificity (%)Negative predictive value (%)Positive predictive value (%)Positive likelihood ratio
  • Group 1, advanced maternal age (AMA)≥ 35 years at expected date of delivery and no other risk factor; Group 2, positive serum screen (any age; positive triple screen with second-trimester Down syndrome risk estimate of ≥ 1/270) or AMA plus a positive family history of a chromosomal abnormality; Group 3, not AMA, normal or unknown maternal serum screen, positive or negative family history, and positive genetic sonogram.

  • *

    Excluding 20 non-Down syndrome chromosomal anomalies.

  • Positive likelihood ratio = Sensitivity/1 − Specificity. NSF, nuchal skinfold.

Increased NSF 15 + 0 to 19 + 6 weeks16.6799.4198.0540.0028.06
Pyelectasis25.0098.5798.5725.0017.44
Short femur16.6798.8998.4222.2214.95
Short humerus16.6798.7398.4120.0013.08
Echogenic bowel16.6798.4198.4116.6710.47
> 1 marker33.3396.3498.6914.819.10
Major congenital anomaly25.0096.5098.5412.007.14
Echogenic intracardiac focus33.3395.2298.6811.766.98
Any 1 marker33.3384.3998.513.922.14

Table 6 contains the calculations for the entire study population analyzed for all chromosomal abnormalities. Major congenital anomalies had the highest PPV and + LR (40.5% and 13.4, respectively), with a sensitivity of 46.9%, specificity of 96.5% and NPV of 97.3%. Increased NSF at 15 + 0 to 19 + 6 weeks had the second highest + LR (12.95). Hypoplastic fifth mid phalanx was eliminated from this table due to lack of significance, with zero sensitivity. The category of one ultrasound marker contained cases with isolated non-anomaly markers, with a low + LR of only 2.6. A low + LR was associated with isolated choroid plexus cysts in this group as well.

Table 6. Efficacy of second-trimester genetic sonogram in total study population (Groups 1–3; n = 660; including 32 with chromosomal abnormalities)
Ultrasound findingsSensitivity (%)Specificity (%)Negative predictive value (%)Positive predictive value (%)Positive likelihood ratio*
  • Group 1, advanced maternal age (AMA)≥ 35 years at expected date of delivery and no other risk factor; Group 2, positive serum screen (any age; positive triple screen with second-trimester Down syndrome risk estimate of ≥ 1/270) or AMA plus a positive family history of a chromosomal abnormality; Group 3, not AMA, normal or unknown maternal serum screen, positive or negative family history, and positive genetic sonogram.

  • *

    Positive likelihood ratio = Sensitivity/1 − Specificity. NSF, nuchal skinfold.

Major congenital anomaly46.8896.5097.2740.5413.38
Increased NSF 15 + 0 to 19 + 6 weeks7.6999.4195.4440.0012.95
Pyelectasis12.5098.5795.6730.778.72
Echogenic bowel12.5098.4195.6728.577.85
> 1 marker21.8896.3496.0323.335.97
Short femur6.2598.8995.3922.225.61
Short humerus6.2598.7395.3820.004.91
Echogenic intracardiac focus15.6395.2295.6814.293.27
Any 1 marker40.6384.3996.5411.712.60
Choroid plexus cyst9.3891.7295.215.451.13

In 141/660 (21.3%) cases the genetic sonogram detected one or more sonographic markers or a major congenital anomaly. After post-sonography counseling, 85 of these 141 (60%) cases chose to undergo genetic amniocentesis. Of these, 17 (20%) were positive for a chromosomal abnormality. Of the 56 cases who declined genetic amniocentesis, three (5.3%) of the neonates were found to have a chromosomal abnormality. This difference was significant (χ2 = 5.95; P = 0.0105; LR χ2 = 6.7, P = 0.010).

In the 79 low-risk cases in which the genetic sonogram detected one or more sonographic markers or a major congenital anomaly (Group 3), 45 (57%) chose to undergo genetic amniocentesis after counseling post-sonography. Of these, seven (15.5%) were positive for a chromosomal abnormality, of which only one was Down syndrome. Of the 34 cases that declined genetic amniocentesis based on the counseling, none of the neonates was found to have a chromosomal abnormality. This difference was significant (χ2 = 5.80; P = 0.016; LR χ2 = 8.4, P = 0.004).

There were four cases of fetal chromosomal abnormalities in the AMA group (Group 1) which did not have genetic amniocentesis in the second trimester, three with Down syndrome and one with 45,X. Two of the Down syndrome fetuses had a normal second-trimester genetic sonogram, one of which was found to have short proximal long bones in the third trimester (long bone biometry < 5th centile for gestational age). The third Down syndrome fetus had sonographic markers detected by second-trimester ultrasound examination and developed hydrops in the third trimester, which led to fetal karyotyping by cordocentesis. In the case with 45,X, the diagnosis was made by third-trimester amniocentesis prompted by the finding of fetal growth restriction.

Discussion

The majority of the medical literature on the second-trimester genetic sonogram has focused on the detection of Down syndrome in large referral centers reporting on primarily high-risk pregnancies1–4, 6, 7. In our study group, which was recruited from two suburban community hospitals, we sought to determine if second-trimester genetic sonograms performed in a community-based antenatal testing unit could be considered an efficient means of screening for clinically relevant fetal chromosomal abnormalities.

The 2001 meta-analysis by Smith-Bindman et al.6, drawn from studies in mostly high-risk pregnancies in specialized referral centers, focused mainly on six isolated sonographic markers for Down syndrome and concluded that although a thickened nuchal fold in the second trimester may be useful in distinguishing unaffected fetuses from those with Down syndrome, the overall sensitivity of this marker was too low to be a practical screening test for Down syndrome2. The detection of increased NSF at 15 + 0 to 19 + 6 weeks in the at-risk population reported here (Table 4) had a sensitivity for detection of Down syndrome of 18.2% and a PPV of 66.7%. Smith-Bindman et al.'s meta-analysis discussed the role of other sonographic markers in the absence of major structural malformations and concluded that they could not discriminate well between unaffected fetuses and those with Down syndrome. Egan et al.16 contended that the meta-analysis of Smith-Bindman et al. revealed that when multiple sonographic markers and structural abnormalities occurred together, sonographic screening sensitivity for Down syndrome was 69% with a false-positive rate of 8%. The importance of detection of congenital anomalies by genetic sonography for Down syndrome screening was also addressed in the studies of Vintzileos and Egan17, 18. In 2007, Smith-Bindman et al.4 concluded, in a prospective population-based cohort study of women at increased risk of a chromosomal abnormality based on the triple screen, that if the normal genetic sonogram were used to counsel these high-risk women that they could avoid amniocentesis, approximately half (115 of 245) of the cases of Down syndrome would have been missed. They reported that, in the absence of a structural anomaly, isolated soft markers were not associated with Down syndrome, with the notable exception of NSF thickening. Yet, Benn and Egan5, commenting on the study of Smith-Bindman et al.4, contended that it actually validated the practice of risk modification by genetic sonogram after positive maternal serum screening.

Recent studies have addressed the role of the genetic sonogram in lower risk populations. Schluter et al.2, in a large, prospective, single-center cohort study from Australia involving 73 Down syndrome cases, found that, after accounting for gestational age and maternal age, all pregnancies with thick NSF, short humerus, echogenic bowel, echogenic intracardiac focus, renal pelvic dilatation and aneuploidy associated anomalies were associated significantly with Down syndrome. Short femur was removed from their models due to colinearity between short humerus and short femur. Malone et al.3 evaluated the role of second-trimester genetic sonography (at 15–23 weeks) in 8533 unselected singleton pregnancies included in the FASTER trial which had already had first-trimester combined screening (nuchal translucency, pregnancy-associated plasma protein-A, free beta-human chorionic gonadotropin (hCG)) and second-trimester Quad screens (alpha-fetoprotein, unconjugated estriol, hCG, inhibin-A). They reported + LRs for major congenital anomalies and markers and concluded that the use of + LRs from second-trimester genetic sonography improves the performance of first- and second-trimester screening by significantly reducing the false-positive rate, with further increases in detection rates for Down syndrome. The highest likelihood ratios were associated with multiple markers, echogenic bowel, major anomaly and NSF thickening.

The multicenter study by Hobbins et al.1 was undertaken to evaluate the diagnostic efficacy of the genetic sonogram in high-risk patients and pooled data from eight centers, involving 176 pregnancies with fetal Down syndrome. One hundred and thirty-four pregnancies were considered high-risk because of AMA and 42 were referred with the indication of a positive triple screen (risk > 1 in 250). Collectively, 71% (125/176) of trisomy 21 affected fetuses were found to have an abnormal long-bone length (femur length, humerus length or both), a major structural abnormality or other sonographically detectable marker. Sensitivities amongst the centers ranged from 63.6% to 80%. The authors concluded that the genetic sonogram can be used to adjust the Down syndrome risk for high-risk patients. The findings reported here add further support to their conclusions.

The study by Wax et al.8 on the efficacy of community-based genetic ultrasonography in detecting chromosomally abnormal fetuses reported that 82% of aneuploid fetuses had an abnormal sonogram (one or more markers present), including 5/7 (71.4%) with Down syndrome, in an exclusively high-risk population of women referred for AMA, positive second-trimester maternal serum screen or prior affected offspring. In contrast, in our essentially high-risk population, we found a 66% (8/12) detection rate for trisomy 21 and a 62.5% (20/32) rate for all chromosomal abnormalities on second-trimester genetic sonography.

In our community hospital-based study we found that the second-trimester sonographic detection of NSF thickening, major congenital anomalies, pyelectasis, short femur, short humerus, echogenic bowel, echogenic intracardiac focus and the presence of more than one marker were significantly associated with Down syndrome (Tables 3 and 4). Choroid plexus cysts were not significantly associated with Down syndrome. This is consistent with previous reports19–21. Similarly, evaluation of the fifth mid phalanx was found to be of no benefit in the detection of second-trimester Down syndrome-affected pregnancies.

When all 660 study subjects were analyzed with regard to all chromosomal abnormalities as the outcome (Table 6), the highest sensitivity (46.9%) was for the detection on genetic sonography of a major congenital anomaly. We did, however, encounter one fetus with trisomy 18 diagnosed only by amniocentesis at 16 weeks, although the sonogram was suboptimal. Sonography at less than 18 weeks may raise the risk of missing this trisomy.

We feel that our experience illustrates two important issues with regard to counseling at-risk women about the role of fetal ultrasound. First, although the detection rate for Down syndrome with the genetic sonogram is fairly high, it is not 100%. Second, the risk of other chromosomal abnormalities also increases with maternal age, but many of these cannot be detected by second-trimester sonography alone. Nevertheless, the NPV of second-trimester sonography with regard to Down syndrome is sufficiently high as to justify confidence in counseling women with a negative genetic sonogram who prefer to avoid genetic amniocentesis and the procedure-related risks. Risk modification based on any screening test, however, remains just an estimate.

A limitation of this study was that, although it was community-based, the majority of our cases were at elevated risk for Down syndrome. Thus, we were unable to provide information on the performance of the genetic sonogram in a low-risk population.

In conclusion, we found that in a community-based antenatal testing unit staffed by MFM subspecialists and RDMS-certified obstetric sonographers, our detection rate for fetal Down syndrome utilizing second-trimester genetic sonography was comparable to the range of sensitivities reported by larger referral centers involving primarily high-risk patients. We feel that it is important for facilities providing fetal genetic sonography to evaluate their own site-specific detection rates to allow more accurate local counseling. Some fetal chromosomal abnormalities, as expected, did not reveal any mid-trimester sonographic abnormalities, but the genetic diagnosis was nevertheless considered to be important information.

Acknowledgements

We wish to express our special gratitude to Peggy McAllister, Darlene Fisler, Lori Mohrman, Deb Schoy, Libby Valenti, Lisa Bottalico, Theresa Waite and Lori White.

Ancillary