Fetal macrosomia and large for gestational age

Abstract

Birthweight has increased in the UK and abroad over the last 30 years, partly attributed to the increasing prevalence of maternal obesity and gestational diabetes. The aim of this review is to provide better understanding of definition, epidemiology, detection and management of the large fetus. 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, defined as weight above the 90th centile by population, customised or international growth charts). Errors in estimation of fetal weight by ultrasound reduce the accuracy of predicting the actual birthweight. Although no single definition is currently universally accepted, the terminology LGA has the advantage of identifying the large fetus even when macrosomia has not yet occurred. Irrespective of definition, fetal overgrowth is associated with an increased risk of adverse perinatal outcomes including need for caesarean delivery, postpartum haemorrhage, third and fourth perineal tears, shoulder dystocia, low Apgar score, admission to neonatal intensive care unit, and increased 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. Previous efforts to prevent fetal overgrowth have had limited success which explain the current focus on improving management once an LGA fetus is identified by ultrasound. Management of LGA has changed substantially in the last decade in response to the ruling Montgomery v Lanarkshire Health Board [2015], national reports from the Healthcare Safety Investigation Branch (HSIB), and international literature. Induction of labour for large for gestational age at early term seems to reduce the incidence of shoulder dystocia but may increase the rate of the third and fourth degree tears. Caesarean section seems to be associated with a reduced risk of LGA related adverse neonatal outcomes, mainly birth trauma, however the number needed to treat is high, being mostly recommended for estimated fetal weight above 4.5 kg in women with diabetes. NICE currently recommends that women with estimated fetal weight above the 95^th centile should have a comprehensive discussion regarding birth options including expectant management, induction of labour and elective caesarean; choice should be offered due to the lack of clear evidence of benefit of one strategy over another. Observational evidence suggests that an estimated fetal weight between the 90^th and the 95^th centile have a much weaker association with adverse neonatal outcomes and is not associated with increased perinatal mortality compared to an estimated fetal weight above the 95^th centile, suggesting discussion regarding mode and timing of birth may not be of benefit between the 90^th and the 95^th centile. There is an ongoing UK randomised controlled trial of induction of labour at early term compared to expectant management for LGA fetuses, which will report soon, and this may help inform the best practice for management of LGA.