Article Text

Download PDFPDF
Accuracy of a mixed effects model interpolation technique for the estimation of pregnancy weight values
  1. Anne Marie Darling1,
  2. Martha M Werler2,3,
  3. David E Cantonwine4,
  4. Wafaie W Fawzi5,
  5. Thomas F McElrath4
  1. 1 Epidemiology, Boston University School of Public Health, Boston, Massachusetts, USA
  2. 2 Department of Epidemiology, Boston University School of Public Health, Boston, Massachusetts, USA
  3. 3 BUSPH Epidemiology
  4. 4 Division of Maternal-Fetal Medicine, Department of Obstetrics and Gynecology, Brigham and Women’s Hospital, Harvard Medical School, Boston, Massachusetts, USA
  5. 5 Department of Epidemiology, Harvard School of Public Health, Boston, Massachusetts, USA
  1. Correspondence to Dr Anne Marie Darling, Epidemiology, Boston University School of Public Health, Boston, MA 02118, USA; annemarie.darling{at}


Background Interpolation of missing weight values is sometimes used in studies of gestational weight gain, but the accuracy of these methods has not been established. Our objective was to assess the accuracy of estimated weight values obtained by interpolating from the nearest observed weight values and by linear and spline regression models when compared with measured weight values.

Methods The study population included participants enrolled in the LIFECODES cohort at Brigham and Women’s Hospital. We estimated weights at 28 (n=764) and 40 (n=382) weeks of gestation using participants’ two nearest observed weights and subject-specific slopes and intercepts derived from repeated measures mixed effects models. In separate models, gestational age was parameterised as a linear and restricted cubic spline variable. Mean differences, absolute error measures and correlation coefficients comparing observed and estimated weights were calculated.

Results Mean differences and mean absolute error for weights derived from the 28-week linear model (0.18 lbs (SD 6.92), 2.73 lbs (SD 6.35)) and 40-week linear model (−0.40 lbs (SD 5.43) and 2.84 lbs (SD 4.65)) were low. Mean differences were somewhat greater at 28 weeks for weight values derived from the nearest two observed values (mean difference −1.97 lbs (SD 8.74)) and from spline models (mean difference −2.25 lbs (SD 7.13)). Results were similar at 40 weeks.

Conclusions Overall, weight values estimated using this interpolation approach showed good agreement with observed values. When repeated measures of weight are available, mixed effects models may be used to interpolate of missing weight values with minimal error.

  • pregnancy
  • weight
  • missing data
  • methods

Statistics from

Request Permissions

If you wish to reuse any or all of this article please use the link below which will take you to the Copyright Clearance Center’s RightsLink service. You will be able to get a quick price and instant permission to reuse the content in many different ways.


  • Contributors All authors contributed to the design of the study and analysis plan. AMD conducted the analysis and drafted the manuscript. All authors provided substantive revisions to the manuscript.

  • Funding This study was supported by T32 HD052458 (Boston University Reproductive, Perinatal and Pediatric Epidemiology training program).

  • Competing interests None declared.

  • Patient consent for publication Not required.

  • Provenance and peer review Not commissioned; externally peer reviewed.

  • Data availability statement Data are available on reasonable request. No data are available.