| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
|
|
|
|||||||
|
|||||||||
|
by the American Institute of Ultrasound in Medicine J Ultrasound Med 21:719-722 • 0278-4297 Maternal Ethnicity and Variation of Fetal Femur Length Calculations When Screening for Down SyndromeDepartment of Obstetrics and Gynecology, Madigan Army Medical Center, Tacoma, Washington. Address correspondence and reprint requests to Christine M. Kovac, MD, Department of Obstetrics and Gynecology, Madigan Army Medical Center, Tacoma, WA 98431.
Objective. To determine whether current methods for detecting Down syndrome based on fetal femur length calculations are influenced by ethnicity. Methods. The study population consisted of all fetuses scanned between 14 and 20 completed weeks' gestation from April 1, 1997, to January 1, 2000. The expected femur length was calculated from the biparietal diameter. The variance from the expected femur length, compared with the biparietal diameter, was calculated, and the mean variations were compared by maternal race. Ethnic-specific formulas for expected femur length were derived by simple regression. Results. There was a statistically significant difference in femur length in the Asian group compared with all other groups, as well as the white group compared with the black and Asian groups (P < .05). However, there was no significant difference between the black and Hispanic groups or the white and Hispanic groups. The Asian group had the largest variation, with the measured femur length being less than the expected femur length. All groups studied had a mean expected femur length less than the mean measured femur length. On the basis of the ethnic-specific formulas for femur length, there was a significant decrease in patients that would undergo further evaluation for Down syndrome. Conclusions. There is a significant difference in the mean expected femur length by biparietal diameter among fetuses in the second trimester with regard to ethnicity. Using ethnic-specific formulas for expected femur length can have a considerable impact on the use of sonographic risk factors for Down syndrome screening. Further data are required for use of femur length as a screening tool in the genetic sonogram.
Key Words: Down syndrome • ethnicity • femur length • sonographic screening Abbreviations: BPD, biparietal diameter • FL, femur length
Sonographic markers are useful in identifying fetuses at increased risk of chromosomal aneuploidy, allowing for counseling of a patient and offering definitive cytogenetic testing. With current techniques, 24% to 93% of fetuses with Down syndrome can be detected with the use of high-resolution sonography in the second trimester.1,2 Some of the sonographic markers currently used include a thickened nuchal fold, echogenic bowel, echogenic intracardiac foci, renal pyelectasis, and shortened long bones such as humeri and femurs.3–7 The fetal femur length (FL) measured in second-trimester sonography has been used as a screening tool for Down syndrome. A measured-expected FL ratio of 0.91 or less is considered an indicator of risk for Down syndrome.6 Published formulas for expected FL exist in the literature,3,6 but each institution's population varies. Established formulas do not necessarily fit each institution's population, and new formulas need to be calculated, as shown by a prior study of FLs by gestational age in our institution.8 Femur length and other sonographic measurements have been shown to vary by race when used with studies of birth weight differences and as genetic screening tools9,10 but not for gestational age dating.11 Other sonographic markers used for screening for Down syndrome have shown variation between races, with a higher prevalence of echogenic intracardiac foci in Asian populations.12 Because of the ethnic diversity in our patient population, we were concerned that certain patient characteristics, such as ethnicity, may influence the fetal FL calculations and may falsely elevate the patient's Down syndrome risk. The goal of our study was to determine whether there was a difference in measured to expected FL in the second-trimester Down syndrome calculations within different ethnic groups and whether there was a difference in deriving formulas for expected FL for each ethnic group.
A prospective evaluation of fetal FLs for all patients undergoing second-trimester sonography between 14 and 20 completed weeks' gestation was performed from April 1, 1997, to January 1, 2000, at the Madigan Army Medical Center Antenatal Diagnostic Center. Aneuploid fetuses were excluded. Madigan Army Medical Center is a tertiary care facility and referral center for military beneficiaries in the Pacific Northwest and Alaska; its obstetric ultrasound diagnostic center has been fully accredited by the American Institute of Ultrasound in Medicine since 1997. Three full-time sonographers with 28 years of combined experience perform all sonographic evaluations in the antepartum diagnostic center in conjunction with maternal-fetal medicine consultation. Sonographic examinations are performed on 1 of 3 machines: 1 HDI 3000 (Philips Ultrasound, Bothell, WA) and 2 128XP (Acuson, a Siemens Company, Mountain View, CA). Sonographic examinations are recorded on an AEGIS database client recording package (Acuson), and a printout is made for the patient's record. Maternal-fetal medicine staff reviews all sonographic examinations. The patient population included both high- and low-risk women, with high risk being defined as age of 35 years or older, pregnancies with abnormal maternal serum analyte screening results, and pregnancies with fetal anomalies. Patients with incomplete records, incomplete fetal measurement data, or fetal deaths at the time of examination were excluded. The dating criteria used to calculate gestational age were based on the patient's last menstrual period if the average age at sonography was within 8 days of the menstrual dates; otherwise, the average age at sonography was used for pregnancy dating. If a patient had earlier sonography that confirmed or changed her dates, this was used as her dating criterion. All patients who had an incomplete fetal anatomic survey were brought back at a later gestational age for reevaluation. Patients were only included once for study purposes. Study subgroups, by maternal self-report of ethnicity, included Asian, black, Hispanic, and white mothers. The expected FL was calculated by the following formula: expected FL = –9.645 + 0.9338 x biparietal diameter (BPD).3 Measured and expected FLs were determined for each ethnic group. The variation from the expected FL was calculated for each group. Then the variation in the FL between ethnic groups was compared by analysis of variance with the Fisher protected least significant difference test for FL difference as the post hoc test. P < .05 was considered significant for this analysis. The FL was plotted against the BPD for each ethnic group, and with a linear-appearing relationship, simple regression was used to calculate the best fit line. We then looked at how many patients would have been identified as being at risk for having a fetus with Down syndrome with a measured-expected FL ratio of 0.91 using the formula of Benacerraf et al3 and our new ethnic-specific formulas. All calculations and statistical analyses were done with StatView for Windows, version 4.5 (SAS Institute Inc, Cary, NC).
There were 980 fetal anatomic surveys included in our study: 63 Asian, 142 black, 59 Hispanic, and 716 white patients. The variance from the expected FL for each group is shown in Table 1  and Figure 1. In a comparison of the expected FL with the measured FL, the Asian group had the largest variation from expected, with the measured FL being less than the expected FL. All groups studied had a mean expected FL less than the mean measured FL, which is evident in the regression curve (Fig. 2).
The difference in the measured and actual FL shown in Figure 1  was compared between ethnic groups, with a significant difference in variation between the Asian and black groups, the Asian and white groups, the Asian and Hispanic groups, and the black and white groups but not the black and Hispanic and Hispanic and white groups (P < .05; Table 2).
The formulas derived for each ethnic group by simple regression analysis of the measured BPD to the measured FL are shown in Table 3 . The numbers of patients who would have had positive results for an increased risk of Down syndrome are shown in Table 4. The overall numbers and the numbers who would have had positive results with our new ethnic-derived formulas are shown. The number of patients who would have had positive results decreased by more than half.
Maternal ethnicity is a factor that may influence detection of fetuses with Down syndrome based on FL calculations. In our patient population, we noted a significant difference in the mean expected FL as calculated by the BPD among fetuses in the second trimester with regard to ethnicity. Shorter femurs with the largest variation from expected were noted among the fetuses of self-identified Asian mothers. All of the ethnic groups in our population, however, had shorter-than-expected FLs when we used the equation of Benacerraf et al.3 Although we considered sonographic technique, our more than 28 years of sonographic experience belies that as a contributory factor, and the discrepancy most likely represents the diversity in our population, making the established formula by Benacerraf et al3 not applicable to our population. Our new ethnic-specific formulas derived from our own Madigan Army Medical Center data, however, had a notable impact on decreasing the number of patients that would require prenatal genetic counseling and potentially invasive diagnostic procedures. On the basis of these data, the number of false-positive diagnoses of short femurs in our population would have decreased by 89. This decrease would have saved that many appointments and up to $90,000 for amniocentesis and karyotyping. This would certainly be beneficial for our patients as well as our resources. If screening of a population is going to be performed, normative data must be calculated from the population to be screened and, as in this case, for the subpopulations within that population. Because aneuploid fetuses were excluded, we were unable to determine the efficacy of screening using the newly derived ethnic-specific formulas. Before our Madigan-specific formulas for calculations of sonographic risk factor assessment are used, a larger cohort of patients should be evaluated, and our ethnic-specific formulas should be validated and evaluated for efficacy. Further investigation with greater numbers of patients is needed.
Received January 31, 2002, from the Department of Obstetrics and Gynecology, Madigan Army Medical Center, Tacoma, Washington. We acknowledge the work of Lori Green, Andrea Heron, and Diane Walkup. The opinions and assertions contained herein are the private views of the authors and are not to be construed as official or reflecting the views of the US Department of the Air Force, Department of the Army, or Department of Defense.
This article has been cited by other articles:
|
||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
Top
Abstract
Introduction
