title: Simple mathematical formulae for estimation of median values of fetal biometry at each gestational age.
abstract: OBJECTIVE
  The aim of this study was to propose simple mathematical formulae to estimate median values of fetal biometry including biparietal diameter (BPD), abdominal circumference (AC) and femur length (FL) at each gestational age (GA) easily without looking up the previously established reference values.
  
  
  METHODS
  Simple mathematical formulae to estimate median values of fetal biometric values at each gestational week were inferred. To validate these formulae, three different linear equations were derived from previously reported reference values of median BPD, AC and FL using regression analysis at each gestational week. Finally, calculated data through the inferred formula were compared to retrospectively collected data (observed data).
  
  
  RESULTS
  The equation revealing the relationship between BPD and GA was: median BPD (cm)=GA (wk)/4. Using this simple mathematical formula, the absolute percentage error between observed data and calculated data ranged from 0.12% to 7.50%. The equation between AC and GA was: median AC (cm)=GA (wk)-5. Through this formula, the absolute percentage error was analyzed same as above and it ranged from 0.30% to 4.76%. Lastly the derived formula between FL and GA was: median FL (cm)=GA (wk)/5 and the absolute percentage error ranged from 4.52% to 16.75%.
  
  
  CONCLUSIONS
  The three simple formulae suggested in our study showed a significantly easy way to estimate the median values of fetal biometry at each gestational week with good reliability.
authors.affiliation: Department of Obstetrics and Gynecology, Seoul National University College of Medicine, Seoul, Korea.
authors.forename: Hwa Young
authors.initials: HY
authors.lastname: Choi
authors.affiliation: Department of Obstetrics and Gynecology, Seoul National University College of Medicine, Seoul, Korea.
authors.forename: Jeong Ha
authors.initials: JH
authors.lastname: Kim
authors.affiliation: Department of Obstetrics and Gynecology, Seoul National University College of Medicine, Seoul, Korea.
authors.forename: Jee Yoon
authors.initials: JY
authors.lastname: Park
authors.affiliation: Department of Obstetrics and Gynecology, Seoul National University Bundang Hospital, Seongnam, Korea.
authors.forename: Eun Young
authors.initials: EY
authors.lastname: Jung
authors.affiliation: Department of Obstetrics and Gynecology, Seoul National University Bundang Hospital, Seongnam, Korea.
authors.forename: Jae Hong
authors.initials: JH
authors.lastname: No
authors.affiliation: Department of Obstetrics and Gynecology, Seoul National University Bundang Hospital, Seongnam, Korea.
authors.forename: Kyung Joon
authors.initials: KJ
authors.lastname: Oh
authors.affiliation: Department of Obstetrics and Gynecology, Seoul National University Bundang Hospital, Seongnam, Korea.
authors.forename: Joon-Seok
authors.identifier: 0000-0002-3832-8445
authors.initials: JS
authors.lastname: Hong
keywords: Biometry; Formula; Mathematical; Ultrasonography