Risk factors for macrosomia and its clinical consequences: a study of 350,311 pregnancies

Objectives: To identify demographic risk factors for either birthweight >4 kg or over the 90th centile and to quantify the obstetric risks. Study design: Data from 350,311 completed singleton pregnancies in the North West Thames Region between 1988 and 1997 were analysed using logistic regression. Predisposing factors and pregnancy outcome were compared by birthweight 2.5–4 kg (n=259,902) and >4 kg (n=36,462) and 10th–90th centile (n=279,780) and >90th centile (n=34,937). Results: Macrosomia defined as birthweight >90th centile was more likely in women whose BMI >30 (kg/m2) (odds ratio (OR) 2.08; confidence intervals (CI) 1.99, 2.17), parity >4 (OR 2.20; CI 2.02, 2.40), age >40 (OR 1.22; CI 1.11, 1.35) and in women with pre-existing diabetes (OR 6.97; CI 5.36, 8.16) or who developed gestational diabetes (OR 2.77; CI 2.51, 3.07). Macrosomia defined by birthweight >4 kg was better than birthweight >90th centile at predicting morbidity and was associated with a prolonged first and second stage of labour (OR 1.57; CI 1.51, 1.63) and (OR 2.03; CI 1.88, 2.19), respectively, an increased risk of instrumental vaginal delivery (OR 1.76; CI 1.68, 1.85), third degree perineal trauma (OR 2.73; CI 2.30, 3.23), emergency caesarean section (OR 1.84; CI 1.75, 1.93), postpartum haemorrhage (OR 2.01; CI 1.93, 2.10), Apgar score <4 (OR 1.35; CI 1.03, 1.76), and admission to the special care baby unit (OR 1.51; CI 21.38, 1.68). Conclusion: Macrosomia is more common in mothers who are obese, older or diabetic and is associated with significant obstetric morbidity.