The utility of customised growth charts for identifying macrosomia and the effect of intervention
Objective: Best management of suspected large for gestational age (LGA) fetuses is unclear. In some hospitals, women with an LGA fetus by customised growth charts are offered earlier induction. This study aimed to examine scan accuracy for this group and the outcome with intervention. Study design: This is a retrospective cohort study of pregnant women taken from 3 groups; women with a suspected LGA fetus, women with diabetes (DM) and a control group of women that underwent induction of labour on or after 280 days gestation. Scan accuracy using GROW and WHO charts in the LGA and DM cohorts was assessed using ROC curves and outcomes between the cohorts were compared. Results: Over a 12 month period 845 suitable cases were identified; LGA (128), DM (116) and control cases (601). Mean gestation at induction was 275.5 (5.8), 270.0 (7.6) and 287.1 (3.3) days. Mean birthweights were significantly different across groups (3818g (345), 3434g (416), 3653g (399)). No excess shoulder dystocia or neonatal morbidity was seen in the LGA group. Spontaneous vaginal delivery was significantly less likely in the LGA group vs the control group at 69/128, 53.9% vs 413/601, 68.7% RR 0.78 (95% CI 0.66-0.93 p0.02). Postpartum haemorrhage >1500mL was more likely in the LGA group, RR 2.28, vs the control group (95% CI 1.41 to 5.62 p<0.01). Mean scan error was -5.2% for the DM group and +15.6% for the LGA group. Positive predict value of scan estimated weight for birthweight >90th centile on GROW chart, >90th centile on WHO chart, and projected birthweight of >4000kg was 0.35-0.40. Of these three thresholds projected birthweight of >4000kg from scan significantly better predicted itself (AUROC 0.80, 0.79 and 0.87). Conclusion: Induction of suspected large for gestational age pregnancies at 39 weeks is an intervention based on a test known to have poor positive predict value. This may increase liabilities and costs without benefit.