PRACTICAL APPROACHES FOR ESTIMATING PREPREGNANT BODY WEIGHT Helen E. Harris, BSc, PhD and George T. Ellison, MSc, PhD ABSTRACT Measurements of prepregnant body weight have important research and clinical applications. In practice, however, they are not always recorded; even when they are, this information is not always readily available. For this reason, researchers and clinicians have to rely on retrospective estimates of prepreg- nant weight, which can be estimated using: 1) maternal self- reports, 2) retrospective extrapolation, or 3) standardized estimates that correct for weight gained during early preg- nancy. The aim of the present study was to examine the relative merits of these three approaches. Maternal self-reports tend to be unreliable and biased, being influenced by a variety of sociodemographic characteristics that generally underesti- mate true prepregnant body weight. Estimates of prepregnant weight based on retrospective extrapolation are vulnerable to measurement error, transient fluctuations in body weight, and incorrectly assume that the rate of weight gain is constant throughout pregnancy. Standardized estimates that correct for weight gained during early pregnancy incorrectly presume that there is little interindividual variation in gestational weight gain and that weight gain is similar for each woman in consecutive pregnancies. Because none of these techniques can provide a precise measure of prepregnant weight, researchers have little alternative but to recruit and weigh women before they become pregnant, although measurements of body weight recorded during the first trimester of pregnancy may provide a reason- able indication of prepregnant weight. For clinicians, self- reports of prepregnant weight or measurements recorded early in pregnancy are probably sufficiently accurate for practical purposes whenever recent, accurate measurements of pre- pregnant weight are unavailable.  1998 by the American College of Nurse-Midwives. Both prepregnant body weight and gestational weight gain have been shown to be associated with the outcome of pregnancy (1,2). In particular, it has been suggested that gestational weight gain is an important risk factor for birth weight, with those mothers who experience low gestational weight gains being more likely to deliver low birth weight neonates (1–3). As a result, the U.S. Institute of Medicine published guidelines for weight gain during pregnancy in 1990 (4) to encourage women to achieve an ‘‘optimal’’ pregnancy outcome. These guidelines are stratified by prepregnant weight, recommending that underweight women (BMI  19.8 kg/m 2 ), normal weight women (BMI, 19.8 –26.0 kg/m 2 ), overweight women (BMI, 26.1–29.0 kg/m 2 ), and obese women (BMI  29.0 kg/m 2 ) gain 12.5–18.0 kg (27.6 –39.7 lb), 11.5–16.0 kg (25.4 –35.3 lb), 7.0 –11.5 kg (15.4 –25.4 lb), and  6.0 kg (13.2 lb), respectively (4)*. Although several researchers have sug- gested that the Institute’s recommendations for gestational weight gain are not sufficiently precise for use as a screen during antenatal care (for either poor obstetric or neonatal outcomes) (5– 8), measurements of prepregnant body weight still provide a good indication of maternal nutritional status prior to conception (4). They are also important for calculating the amount of weight gained during pregnancy (9) so that compliance with the Institute’s guidelines (4) can be assessed and appropriate targets for ‘‘optimal’’ gesta- tional weight gain can be selected (4,10). As such, prepreg- nant weight remains an important anthropometric mea- sure for both clinicians and researchers alike. In those settings where existing medical records contain recent, accurate measurements of prepregnant maternal weight, researchers and clinicians should use these. How- ever, in practice, these records often are unavailable and when they are available, recent accurate measurements of prepregnant body weight are not always routinely recorded (11). Under these circumstances, researchers and clinicians have to rely on retrospective estimates of prepregnant body weight, and there are three techniques that can be used to generate these estimates. The aim of the present study was to examine the relative merits of these three approaches and to provide practical suggestions for use in research and clinical settings. TECHNIQUES FOR RETROSPECTIVELY ESTIMATING PREPREGNANT BODY WEIGHT Prepregnant body weight can be estimated retrospec- tively using 1) maternal self-reports, 2) retrospective extrapolation, and 3) standardized estimates that correct for weight gain during early pregnancy. Address correspondence to Helen E. Harris, BSc, PhD, Communica- ble Disease Surveillance Centre, Public Health Laboratory Service, 61 Colindale Avenue, London NW9 5EQ, United Kingdom. * Body mass index (BMI) is calculated by dividing weight (measured in kg) by the square of height (measured in meters). To calculate BMI from American units of measure, convert weight to kg (lb  0.454), convert height to meters (in.  0.024) then apply the formula weight/height 2 . Journal of Nurse-Midwifery • Vol. 43, No. 2, March/April 1998 97  1998 by the American College of Nurse-Midwives 0091-2182/98/$19.00 • PII S0091-2182(97)00159-6 Issued by Elsevier Science Inc.