RELATIVE MACROSOMIA IDENTIFIED BY THE INDIVIDUALIZED BIRTH-WEIGHT RATIO (IBR) - A BETTER METHOD OF IDENTIFYING THE AT RISK FETUS

Objective. To assess the effectiveness of a newly developed individualised birthweight ratio (IBR), which corrects for physiological birthweight determinants, in identifying infants at risk from the complications of macrosomia. Design. Prospective observational study. Setting. Obstetric unit, Nottingham City Hospital. Subjects. 2835 women delivered between December 1991 and July 1992 and the infants of 624 of these, selected by virtue of their birthweight for gestation and IBR centile positions. Main outcome measures. Skinfold thickness and ponderal index measurements, operative delivery, shoulder dystocia, fetal trauma, impaired glucose tolerance. Results. Using an IBR above the 90th centile as a cut off results in 2.4% of infants being reclassified as normally grown and 3.1% are reclassified as large. The IBR des not result in the identification of any more infants with abnormal ponderal indices or skinfold thicknesses than birthweight for gestation. It does, however, identify more of the infants at risk of operative delivery, shoulder dystocia, fetal trauma and impaired glucose tolerance. Conclusion. The IBR significantly improves upon birthweight for gestation in identifying infants who suffer from the complications of relative macrosomia.