The macrosomic fetus: a challenge in current obstetrics

Background. There has been a rise in the prevalence of large newborns over a few decades in many parts of the world. There is ample evidence that fetal macrosomia is associated with increased risk of complications both for the mother and the newborn. In current obstetrics, the macrosomic fetus represents a frequent clinical challenge. Evidence is emerging that being born macrosomic is also associated with future health risks. Objective. To provide a review of causes and risks, prevention, prediction and clinical management of suspected large fetus/fetal macrosomia, primarily aimed at clinical obstetricians. Methods. Medline and EMBASE were searched between 1980 and 2007 by combining either 'fetal macrosomia' or 'large for gestational age' with other relevant terms. The Cochrane Database of Systematic Reviews was searched for the term 'fetal macrosomia'. Results and conclusions. Although the causes of high birthweight include both genetic and environmental factors, the rapid increase in the prevalence of large newborns has environmental causes. The evidence is extensive that maternal overweight and associated metabolic changes, including type 2 and gestational diabetes, play a central role. There is a paucity of studies of the effect of intervention before and/or during pregnancy on the risk of having an 'overweight newborn'. It appears rational, however, that preventive measures should primarily be implemented before pregnancy and should include guidance about nutrition and physical activity in order to reduce the prevalence of overweight. In pregnancy, limited weight gain, especially in obese women, seems to reduce the risk of macrosomia, as do good control of plasma glucose among those with diabetes. Prediction of fetal macrosomia remains an inaccurate task even with modern ultrasound equipment. There is little evidence that routine elective delivery (induction or caesarean section) for the mere reason of suspected macrosomia should be employed in a general population. Vaginal delivery of a macrosomic fetus requires considered attention by an experienced obstetrician and preparedness for operative delivery, shoulder dystocia and newborn asphyxia.