Fetal macrosomia

Birthweight has increased in the last century partly attributed to increasing maternal obesity and prevalence of gestational diabetes. Although changes in mean birthweight reflect the trends and demography of the population, increased risk of adverse outcomes occurs at the extremes of the distribution (i.e. in small and large infants). The aim of this review is to provide better understanding of definition, epidemiology, prediction and management of large infants. Many definitions of large infants, or fetal overgrowth, have been described in the literature, including macrosomia (weight above 4 kg) or large for gestational age (LGA) (weight above the 90th centile by population, customized or international charts). However, no single definition is currently universally accepted. Irrespective of definition, fetal overgrowth is associated with increased risk of adverse perinatal outcomes, including need for caesarean delivery, postpartum haemorrhage, severe perineal tears, low Apgar score, admission to neonatal intensive care unit, and severe neonatal morbidity and perinatal mortality. Major risk factors for LGA are maternal obesity, diabetes and increased gestational weight gain but these are not highly predictive of LGA. Efforts to prevent fetal overgrowth have had limited success, helping to justify the current focus on improving management once a large infant is identified by ultrasound. Induction of labour for the large for gestational age fetus at term is a promising strategy but ongoing studies will help inform timing of induction and target population. The Royal College of Obstetricians and Gynaecologists (RCOG) suggests elective caesarean section should be considered when the estimated fetal weight is above 4.5 kg in women with diabetes. Although caesarean section should not be recommended below this threshold, the ruling Montgomery v Lanarkshire Health Board [2015] has emphazised our duty to provide clear and comprehensive information during antenatal care.