title: Is It Time to Change Our Reference Curve for Femur Length? Using the Z-Score to Select the Best Chart in a Chinese Population.
abstract: OBJECTIVE
  To use Z-scores to compare different charts of femur length (FL) applied to our population with the aim of identifying the most appropriate chart.
  
  
  METHODS
  A retrospective study was conducted in Beijing. Fifteen hospitals in Beijing were chosen as clusters using a systemic cluster sampling method, in which 15,194 pregnant women delivered from June 20th to November 30th, 2013. The measurements of FL in the second and third trimester were recorded, as well as the last measurement obtained before delivery. Based on the inclusion and exclusion criteria, we identified FL measurements from 19996 ultrasounds from 7194 patients between 11 and 42 weeks gestation. The FL data were then transformed into Z-scores that were calculated using three series of reference equations obtained from three reports: Leung TN, Pang MW et al (2008); Chitty LS, Altman DG et al (1994); and Papageorghiou AT et al (2014). Each Z-score distribution was presented as the mean and standard deviation (SD). Skewness and kurtosis and were compared with the standard normal distribution using the Kolmogorov-Smirnov test. The histogram of their distributions was superimposed on the non-skewed standard normal curve (mean = 0, SD = 1) to provide a direct visual impression. Finally, the sensitivity and specificity of each reference chart for identifying fetuses <5th or >95th percentile (based on the observed distribution of Z-scores) were calculated. The Youden index was also listed. A scatter diagram with the 5th, 50th, and 95th percentile curves calculated from and superimposed on each reference chart was presented to provide a visual impression.
  
  
  RESULTS
  The three Z-score distribution curves appeared to be normal, but none of them matched the expected standard normal distribution. In our study, the Papageorghiou reference curve provided the best results, with a sensitivity of 100% for identifying fetuses with measurements < 5th and > 95th percentile, and specificities of 99.9% and 81.5%, respectively.
  
  
  CONCLUSIONS
  It is important to choose an appropriate reference curve when defining what is normal. The Papageorghiou reference curve for FL seems to be the best fit for our population. Perhaps it is time to change our reference curve for femur length.
authors.affiliation: Department of Obstetrics and Gynecology, Peking University First Hospital, Beijing, China.
authors.forename: Boya
authors.identifier: http://orcid.org/0000-0002-9402-1608
authors.initials: B
authors.lastname: Li
authors.affiliation: Department of Obstetrics and Gynecology, Peking University First Hospital, Beijing, China.
authors.forename: Huixia
authors.initials: H
authors.lastname: Yang
authors.affiliation: Department of Obstetrics and Gynecology, Peking University First Hospital, Beijing, China.
authors.forename: Yumei
authors.initials: Y
authors.lastname: Wei
authors.affiliation: Department of Obstetrics and Gynecology, Peking University First Hospital, Beijing, China.
authors.forename: Rina
authors.initials: R
authors.lastname: Su
authors.affiliation: Department of Obstetrics and Gynecology, Peking University First Hospital, Beijing, China.
authors.forename: Chen
authors.initials: C
authors.lastname: Wang
authors.affiliation: Tongzhou Maternal and Child Health Hospital of Beijing, Beijing, China.
authors.forename: Wenying
authors.initials: W
authors.lastname: Meng
authors.affiliation: Peking University Third Hospital, Beijing, China.
authors.forename: Yongqing
authors.initials: Y
authors.lastname: Wang
authors.affiliation: General Hospital of Beijing Military Region, Beijing, China.
authors.forename: Lixin
authors.initials: L
authors.lastname: Shang
authors.affiliation: Center Hospital of Aviation Industry, Beijing, China.
authors.forename: Zhenyu
authors.initials: Z
authors.lastname: Cai
authors.affiliation: Pinggu Maternal and Child Health Hospital of Beijing, Beijing, China.
authors.forename: Liping
authors.initials: L
authors.lastname: Ji
authors.affiliation: Beijing Hospital of Miyun City, Beijing, China.
authors.forename: Yunfeng
authors.initials: Y
authors.lastname: Wang
authors.affiliation: Navy General Hospital, Beijing, China.
authors.forename: Ying
authors.initials: Y
authors.lastname: Sun
authors.affiliation: Beijing Daxing District Hongxing Hospital, Beijing, China.
authors.forename: Jiaxiu
authors.initials: J
authors.lastname: Liu
authors.affiliation: Beijing Chui Yang Liu Hospital, Beijing, China.
authors.forename: Li
authors.initials: L
authors.lastname: Wei
authors.affiliation: Peking University Shougang Hospital, Beijing, China.
authors.forename: Yufeng
authors.initials: Y
authors.lastname: Sun
authors.affiliation: Combined with Traditional Chinese and Western Medicine Hospital of Beijing City, Beijing, China.
authors.forename: Xueying
authors.initials: X
authors.lastname: Zhang
authors.affiliation: Beijing No.6 Hospital, Beijing, China.
authors.forename: Tianxia
authors.initials: T
authors.lastname: Luo
authors.affiliation: Beijing Changping Hospital of Traditional Chinese Medicine, Beijing, China.
authors.forename: Haixia
authors.initials: H
authors.lastname: Chen
authors.affiliation: General Hospital of Jingmei Group, Beijing, China.
authors.forename: Lijun
authors.initials: L
authors.lastname: Yu
mesh_terms: D000328:Adult; D002681:China; D005260:Female; D005269:Femur / Q000000981:diagnostic imaging / Q000196:embryology*; D006801:Humans; D011247:Pregnancy; D012016:Reference Values; D016216:Ultrasonography, Prenatal / Q000592:standards*