The individualised birthweight ratio: a more logical outcome measure of pregnancy than birthweight alone

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Abstract
Objective To provide a new outcome measure for pregnancy specifically related to the individual. Design Computer analysis of physiological factors affecting birthweight. Setting Two provincial teaching hospitals (University and City Hospitals, Nottingham) and an associated district general hospital (Derby City Hospital) serving a defined catchment area in the East Midlands. Subjects All women delivering in the above hospitals since the start of computerised obstetric records: 31 561 women with gestational age verified by early pregnancy ultrasound scan data. Main Outcome Measures Calculation of the predicted birthweight taking into account maternal and fetal physiological factors. Derivation of the individualised birthweight ratio (actual birthweight divided by predicted birthweight expressed as a percentage) for each individual baby. Results The individualised birthweight ratio redefines as normally grown 41% of babies below the 10th centile of crude birthweight for gestation. Other babies previously regarded as normal are redefined as growth retarded. At the upper end of the distribution 46% of those above the 90th centile of birthweight for gestation are redefined as normally grown. Conclusions The predicted birthweight can be calculated for an individual pregnancy at a given gestation. The standardised comparison between this predicted birthweight and the actual birthweight is a more logical reflection of the normality of intrauterine growth and therefore more logical as an outcome measure for pregnancy than crude birthweight for gestation.

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