Background：
To investigate normal reference values of nuchal translucency (NT) thickness in normal Taiwanese fetuses between 11 and 14 weeks of gestation.
Methods：
A prospective study of ultrasound measurements of fetal NT and crown-rump length (CRL) at 11-14 weeks of gestation was conducted in 724 consecutive Taiwanese fetuses between 1998 and 2001. The relationship between NT and 5-mm intervals of the CRL of the fetus was analyzed. NT thickness was converted into multiple of median (MoM) values for the proper CRL. The estimated risk of trisomy 21 was calculated in combination with maternal age and NT MoM
Results：
.NT thickness increased with increasing CRL and gestational week in the first trimester. The mean (median) of NT thickness at 11-14 weeks was 1.56 (1.50) mm. Values of NT logMoM showed a normal Gaussian distribution with a mean of -0.0062 and standard deviation of 0.1146. The overall frequency of NT thickness of >2.5 mm and >3.0 mm was 1.7% (12/724) and 0.7% (5/724), respectively. There were 18 (2.5%) of 724 normal fetuses with the eseimated risk of trisomy 21, based on maternal age and NT thickness higher than 1:300.
Conclusion：
Because of weekly variations and racial differences in NT measurements, normal reference values should be established to convert NT thickness into MoM values for calculating the estimated risk of trisomy 21 in first-trimester NT screening.
(Chang Gung Med J 2003;26:12-9)
Keywords：
nuchal translucency, crown-rump length (CRL), multiple of median (MoM), Down's syndrome screening.

METHODS

Between April 1998 and December 2001, a prospective observational study was conducted to investigate normal reference values of NT thickness in Taiwanese fetuses at 11-14 weeks of gestation. Demographic details and sonographic findings, including the number of fetuses, CRL, and NT thickness, were entered into a computer database at the time of scanning. The gestational age was determined by the last menstrual period (LMP). Those cases with an uncertain date for the LMP, multiple pregnancies, intrauterine fetal death, or fetal anomalies were excluded from this study. All fetuses in this study were followed up and examined carefully after birth.
Ultrasound measurements were performed by 1 of the authors (J.J. Hsu) who is certificated by the Fetal Medicine Foundation in London. A curvilinear 5-MHz transabdominal transducer (Acuson 128XP/10, Acuson, Mountain View, CA, USA) was used for the NT and CRL measurements with data recording that allowed precision to 0.1 mm. Measurements of NT and CRL were performed according to the guidelines provided by the Fetal Medicine Foundation.(9) In our cases, NT thickness was measured on a magnified image (Fig. 1A), a view focused on the fetal neck area to clarify the NT margin, instead of using a standard image (Fig. 1B) in which the fetal image occupies about 3/4 of the screen as suggested by the Fetal Medicine Foundation. The maximal thickness of the subcutaneous translucency between the skin and the soft tissue overlying the cervical spine was measured in an exact sagittal section when the fetus was in a sagittal neutral position. Care was taken to distinguish between fetal skin and the amnion membrane due to the similar appearance of both structures at this gestational age. Calipers were placed directly on the border of the echogenic to non-echogenic tissue ('on the line'). The images were subjected to regular internal auditing by the Fetal Medicine Foundation to verify the standardization and distribution of measurements.
We subdivided all cases into 7 categories according to 5-mm intervals of the CRL of the fetuses. CRL-specific medians for NT thickness were calculated by a weighted non-linear regression from the observed medians of each CRL category. To allow for weekly variations, the results of all data were converted into multiple of the median (MoM) values for the proper CRL. For each pregnancy, the estimated risk of trisomy 21 was calculated from the maternal age and gestational age-related prevalence of trisomy 21, which was then multiplied by the likelihood ratio from the NT measurement. The likelihood ratio is proportional to the degree of deviation in NT thickness from the normal-the thicker the nuchal translucency, the higher the risk of trisomy 21.(5) In this study, the estimated risk of trisomy 21 in each case was calculated using software provided by the Fetal Medicine Foundation.
Means and standard deviations (SD) were used for descriptive purposes. We used the Kolmogorov-Smirnov test to assess the normal distributions of the NT MoM values and used Pearson correlation coefficients to assess correlations among these various factors. A p value of ?.05 was considered statistically significant. Statistical analyses were performed using the statistical software package SPSS for Windows Release 9.0.0 (Chicago, IL).

RESULTS

The study population consisted of 808 singleton pregnancies attending for routine antenatal care or prenatal diagnosis. After excluding 45 cases with uncertain LMP or those lost to follow-up, 23 cases of twin pregnancies, and 16 cases with chromosomal abnormalities, a total of 724 singleton pregnancies was enrolled in this study. A significant correlation was found between CRL measurements and gestational age (r=0.556, p<0.0001). The ranges of gestational age, CRL, and maternal age were 11.2-13.9 weeks, 39.8-84.1 mm, and 18.5-42.8 years, respectively, in this study population.
Mean (SD) and median values of NT thickness for each category of CRL are shown in Table 1. Mean (SD) NT thickness and its MoM values were 1.56 (0.41) (median, 1.5) mm and 1.02 (0.28) (median, 0.99) MoM, respectively. We found that increased NT thickness was associated with increasing CRL of the fetus at 11-14 weeks of gestation (Table 1). From regression analysis on the observed medians of CRL and NT thickness in each CRL category, we found that the best fitting equation was log-quadratic (Table 2): logNT= -0.621139+0.019070 CRL-0.000101CRL2, (r2=0.998). Table 3 demonstrates the percentiles of NT thickness according to 5-mm intervals of the CRL of fetuses. The median of NT measurements increased from 1.10 mm with a CRL<50 mm to 1.75 mm in the category with a CRL?5 mm. Similarly, the 95th percentiles of NT measurements increased from 2.05 mm with a CRL<50 mm to 2.56 mm in the category with a CRL?5 mm. Figure 2 depicts the distribution of NT thickness measurements for the relevant gestational weeks.
There was no significant correlation between maternal age and NT logMoM values (r=0.006, p=0.873). The NT logMoM values showed a log Gaussian distribution (D=0.925 and p=0.359) with a mean of -0.0062 and an SD of 0.1146. Table 4 shows the distribution of NT measurements at 5-mm intervals of the CRL of fetuses. The overall frequency of NT measurements ?.5 mm and ?.0 mm was 1.7% (12/724) and 0.7% (5/724), respectively. In the category with CRL <50 mm, 5.7% and 2.6% of the study group had NT measurements ?.5 mm or ?.0 mm, respectively. At a CRL ?5 mm, 4.8% and 1.8% of the study group had NT measurements ?.5 mm or ?.0 mm, respectively. The estimated risk of trisomy 21, based on maternal age and NT thickness, was 1 in 300 or higher in 18 (2.5%) fetuses of this study population (Table 5).

DISCUSSION

Little is known of the physiological basis which can explain increased in NT thickness in either some of normal and abnormal fetuses during the first trimester of pregnancy. It may represent an accumulation of fluid associated with overperfusion for protecting the developing fetal neural structures.(17) The placenta grows rapidly at the end of the first trimester with a consequent increase in circulating blood volume in parallel with that of fetal circulation.(18) Between 9 and 12 weeks of gestation, the bony components over the posterior neck are not fully differentiated, leading to the formation of transient physiological edema at the level of the nuchal fold to protect intracranial organs from the risk of overperfusion.(17)
In addition, increased NT is recognized in fetuses with cardiovascular malformations. There is a strong association between increased NT and Down's syndrome in humans and animals.(19) Moreover, it is well known that Down's syndrome is often accompanied by a great incidence of cardiovascular malformations.(20) Fetuses with normal karyotypes, but affected by cardiovascular malformations, do show increased NT in ultrasound examinations during the first trimester.(10) In particularly, there is an association between the degree of nuchal edema and the severity of the cardiovascular disease.(17)
Our study confirms previous reports which found that fetal NT thickness appears to increase with gestational age.(3,5,13,15) Therefore, it is not appropriate to use a fixed cutoff point for NT thickness regardless of the gestational age in NT screening for Down's syndrome. When determining a given increase in NT thickness, it is essential to take the gestational age or the CRL into account. Measuring the CRL is commonly used to calculate the gestational age during the first trimester. There are 2 reasons to consider using CRL instead of gestational age for NT screening. First, it is common for pregnant women to be uncertain of the date of the LMP; and second, it is convenient to measure the CRL of the fetus at the same time that the NT thickness is measured.
An increased false-positive rate (NT thickness ?.5 mm or ? mm) with increasing gestational age was noted in previous reports.(3,13,21) However, various frequencies of increased NT thickness in different categories of CRL were found in the present study. This finding confirms that the concept of a fixed cutoff point is not clinically valid. In addition, the false-positive rate in our result (1.7%) was far lower than those of Jou et al.(21) (6.3%), Scott et al.(13) (5.5%), and Pandya et al.(12) (5.7%) who used a cutoff point of 2.5 mm. This may be because we measured NT thickness on a magnified image on which the NT margin could be clearly identified and the thickness accurately measured. Differences of even a few decimal millimeter can influence the results of the NT-based likelihood ratio and the estimated risk of trisomy 21. Thus, accurate measurement of NT thickness is mandatory for the assessment of risk of trisomy 21 by ultrasound. Although a suitable uniform protocol has been established by the Fetal Medicine Foundation,(9) it seems better to use a magnified image in order to accurately measure NT thickness.
Whitlow and Economides(22) demonstrated that the optimal time to examine fetal anatomy and measure NT thickness in the first trimester is 13 weeks. The success rate of visualizing the complete fetal anatomy is increased progressively from 10 to 14 weeks. At 12-13 weeks, the complete anatomy could be clearly visualized in 96%-98% of fetuses.(22,23) The success rate of measuring NT was similar at 10-13 weeks but significantly less at 14 weeks. Braithwaite et al.(23) reported that the NT increases to a maximum at 13 weeks+2 days of gestation. A steady decrease and then disappearance of the NT was found after 14 weeks of gestation. This may account for the necessity to measure NT before 14 weeks of gestation. In addition, increased NT thickness in affected and unaffected fetuses may either resolve or evolve into nuchal edema in the second trimester. Thus, the resolution of NT by the second trimester does not reduce the risk for fetal trisomy and should not falsely reassure clinicians or patients.(24)
Currently, it is common to use the concept of MoM to express the relationship between NT thickness and gestational age in NT screening for Down's syndrome.(5,25,26) Since there is no significant difference in CRL between Asian and Caucasian populations,(27) expressing the NT measurements in MoM may eliminate differences due to race and institution.(21,26) Wide variations in results of NT screening may be attributed to different statistical methods as well as race-specific distributions. The report by Thilaganathan et al.(16) challenged every center to establish their own database for the NT screening for Down's syndrome.
A new method for assessment risk of trisomy 21 uses a combination of maternal age-specific risk and the MoM value of the NT thickness measurement. A cutoff for estimated trisomy 21 risk of 1 in 300 has been suggested for NT screening in the first trimester.(5) Snijders et al. revealed that 8.3% of normal pregnancies and 82.2% of those with trisomy 21 had an estimated trisomy 21 risk of 1 in 300 or higher. Therefore, for a cutoff value of estimated trisomy 21 risk of 1 in 300, the sensitivity was 82.2% and the false-positive rate was 8.3%.(5) However, our population showed that only 2.5% of the normal pregnancies had an estimated trisomy 21 risk of 1 in 300 or higher. This may be attributed to racial differences and to taking the NT measurement from the magnified image.
Measurement of NT thickness has been documented as an effective method of first-trimester screening for fetal chromosomal abnormalities. However, there are limited data dealing with the NT thickness in Taiwanese populations. Our data may provide preliminary information to perform first-trimester NT screening for Down's syndrome in Taiwan. We recommended that one should use the estimated risk of trisomy 21, instead of a fixed cutoff point of NT thickness, based on center-specific median MoM values derived from CRL. Nevertheless, NT screening for Down's syndrome should be applied under expert training and internal auditing in the future.

REFERENCES

1.Nicolaides KH, Azar G, Byrne D, Mansur C, Marks K. Fetal nuchal translucency: ultrasound screening for chromosomal defects in first trimester of pregnancy. Br Med J 1992;304:867-9.
2. Nicolaides KH, Brizot ML, Snijders RJ. Fetal nuchal translucency thickness: ultrasound screening for fetal trisomy in the first trimester of pregnancy. Br J Obstet Gynaecol 1994;101:782-6.
3. Brambati B, Cislaghi C, Tului L, Alberti E, Amidani M, Colombo U, Zuliani G. First-trimester Down's syndrome screening using nuchal translucency: a prospective study in patients undergoing chorionic villus sampling. Ultrasound Obstet Gynecol 1995;5:9-14.
4. Wald NJ, George L, Smith D, Densem JW, Petterson K. Serum screening for Down's syndrome between 8 and 14 weeks of pregnancy. Br J Obstet Gynaecol 1996;103:407-12.
5. Snijders RJM, Noble P, Sebire N, Souka A, Nicolaides KH. UK multicentre project on assessment of risk of trisomy 21 by maternal age and fetal nuchal-translucency thickness at 10-14 weeks of gestation. Lancet 1998;352: 343-6.
6. Hsu JJ, Chiu CH, Hsieh CC, Hsieh TT. The application of fetal nuchal translucency in the detection of chromosomal abnormalities during the first trimester in an Asian population. Ultrasound Obstet Gynecol 1997;14(suppl 1):252.
7. Jauniaux E, Brown R, Snijders RJM, Noble P, Nicolaides KH. Early prenatal diagnosis of triploidy. Am J Obstet Gynecol 1997;176:550-4.
8. Wald NJ, Hackshaw AK, Watt H. Nuchal translucency and trisomy 18. Prenat Diagn 1999;19:995-6.
9. Snijders RJ, Johnson S, Sebire NJ, Noble PL, Nicolaides KH. First-trimester ultrasound screening for chromosomal defects. Ultrasound Obstet Gynecol 1996;7:216-26.
10. Hyett J, Perdu M, Sharland G, Snijders R, Nicolaides KH. Using fetal nuchal translucency to screen for major congenital cardiac defects at 10-14 weeks of gestation: population based cohort study. Br Med J 1999;318:81-5.
11. Souka AP, Snijders RJM, Novakov A, Soares W, Nicolaides KH. Defects and syndromes in chromosomally normal fetuses with increased nuchal translucency thickness at 10-14 weeks of gestation. Ultrasound Obstet Gynecol 1998;11:391-400.
12. Pandya PP, Snijders RJ, Johnson S, Brizot ML, Nicolaides KH. Screening for fetal trisomies by maternal age and fetal nuchal translucency thickness at 10 to 14 weeks of gestation. Br J Obstet Gynaecol 1995;102:957-62.
13. Scott F, Boogert A, Sinosich M, Anderson J. Establishment and application of a normal range for nuchal translucency across the first trimester. Prenat Diagn 1996;16:629-34.
14. Pajkrt E, Mol BWJ, van Lith JMM, Bleker OP, Bilardo CM. Screening for Down's syndrome by fetal nuchal translucency measurement in a general obstetric population. Ultrasound Obstet Gynecol 1998;12:163-9.
15. Pajkrt E, de Graaf IM, Mol BWJ, van Lith JMM, Bleker OP, Bilardo CM. Weekly nuchal translucency measurements in normal fetuses. Obstet Gynecol 1998;91:208-11.
16. Thilaganathan B, Khare M, Williams B, Wathen NC. Influence of ethnic origin on nuchal translucency screening for Down's syndrome. Ultrasound Obstet Gynecol 1998;12:112-4.
17. Moscoso G. Fetal nuchal translucency: a need to understand the physiological basis. Ultrasound Obstet Gynecol 1995;5:6-8.
18. Jauniaux E, Burton GL, Moscoso G. Development of the early placenta: a morphometric study. Placenta 1991; 12:269-76.
19. Vuillemin M, Pexider T, Winking H. Pathogenesis of various forms of double outlet right ventricle in mouse fetal trisomy 13. Int Cardio J 1991;33:281-304.
20. DeVore GR. Trisomy 21: 91% detection rate using second-trimester ultrasound markers. J Ultrasound Med 2001;16:133-41.
21. Jou HJ, Wu SC, Li TC, Hsu HC, Tzeng CY, Hsieh FJ. Relationship between fetal nuchal translucency and crown-rump length in an Asian population. Ultrasound Obstet Gynecol 2001;17:111-4.
22. Whitlow BJ, Economides DL. The optimal gestational age to examine fetal anatomy and measure nuchal translucency in the first trimester. Ultrasound Obstet Gynecol 1998;11:258-61.
23. Braithwaite JM, Armstrong MA, Economides DL. Assessment of fetal anatomy at 12 to 13 weeks of gestation by transabdominal and transvaginal sonography. Br J Obstet Gynaecol 1996;103:82-5.
24. Pandya PP, Snijders RJ, Johnson S, Nicolaides KH. Natural history of trisomy 21 fetuses with increased nuchal translucency thickness. Ultrasound Obstet Gynecol 1995;5:381-3.
25. Spencer K, Souter V, Tul N, Snijders R, Nicolaides KH. A screening program for trisomy 21 at 10-14 weeks using fetal nuchal translucency, maternal serum free b-human chorionic gonadotropin and pregnancy-associated plasma protein-A. Ultrasound Obstet Gynecol 1999;13:1-7.
26. Schuchter K, Wald N, Hackshaw AK, Hafner E, Liebhart E. The distribution of nuchal translucency at 10-13 weeks of pregnancy. Prenat Diagn 1998;18:281-6.
27. Parker AJ, Davies P, Newton JR. Assessment of gestational age of the Asian fetus by the sonar measurement of crown-lump length and biparietal diameter. Br J Obstet Gynaecol 1982;89:836-8.

From the Department of Obstetrics and Gynecology, Chang Gung Memorial Hospital, Taipei.
Received: Feb. 28, 2002; Accepted: Aug. 30, 2002
Address for reprints: Dr. Jenn-Jeih Hsu, Department of Obstetrics and Gynecology, Chang Gung Memorial Hospital. 199, Tung-Hwa North Road, Taipei 105, Taiwan, R.O.C. Tel.: 886-2-27135211 ext. 3345; Fax: 886-2-27197368; E-mail: jjhsu@ms6.hinet.net