A gestational-age-independent model of birth weight based on placental size.

A placental weight (P) to birth weight (W) relationship, W = P1.5 x constant (A), was presented in 1966. However, based on histological and theoretical studies, placental efficiency increases proportionally to the square of the gestational age in weeks. The purpose of this study was to develop and validate a gestational-age-independent model of birth weight based on P2. Data were prospectively entered into a computerized database for liveborn, structurally normal neonates at a tertiary, urban center. 29,989 neonates had gestational ages based on menstrual dating, ultrasound and Ballard examination, 81.4% were African American. The 'best fit' of W = A x P1.5 had errors in predicting the mean birth weight by at least 10% from 24 to 33 weeks of gestation. In contrast, there was significant improvement (p < 0.05) when the model W = 0.0071 x P2 was used; errors of greater than 10% only occurred at 28 and 43 weeks of gestation. This model had similar accuracy when stratified according to gender or ethnicity. A gestational-age-independent model of mean birth weight can be based upon the square of placental weight. We hypothesize that this model may have utility in understanding disorders of intrauterine fetal growth.