Fetal macrosomia
Overview
The most common threshold that has been proposed for macrosomia is estimated fetal weight above 4,000 g, as the risks of complications for infants and mothers are greater past this point. These risks further increase when estimated fetal weight is beyond 4,500 g, which ACOG currently uses to define macrosomia.1 Instead of using this specific cut-off, however, Boulet and colleagues proposed a grading system: grade 1 for fetal weight 4,000 to 4,499 g, grade 2 for fetal weight 4,500 to 4,999 g, and grade 3 for fetal weight over 5,000 g.2 Of course, it's important to distinguish fetal macrosomia from "large for gestational age," which implies estimated fetal weight (EFW) greater than or equal to the 90th percentile for a given gestational age. The reported 90th percentile for birthweight at 37, 38, 39, 40, and 42 completed weeks of gestation are 3,755, 3,867, 3,980, 4,060, and 4,098 g, respectively.1
Causes
Causes for macrosomia include factors that contribute to excessive fetal growth and weight gain. * Diabetes that is poorly controlled in pregnancy is the greatest risk factor for fetal macrosomia. This is believed to be partially explained by excessive growth due to elevated maternal plasma glucose levels and resulting elevated insulin and insulinlike growth factor levels, which stimulate glycogen synthesis, fat deposition, and fetal growth. * Excessive maternal weight gain and/or prepregnancy weight also play the some role in macrosomia by providing excessive growth in selected cases. Whether this is also due to undiagnosed glucose intolerance in these individuals remains to be studied. * Genetic factors also contribute to fetal size. Taller and heavier parents typically produce larger offspring.
Prevention
With the exception of optimal blood glucose management in pregnancies complicated by diabetes, little is known about the prevention of macrosomia. The association between maternal weight, weight gain during pregnancy and macrosomia has led to a proposal that the optimization of maternal weight before pregnancy and limitation of weight gain during pregnancy would be useful strategies.36 The impact of maternal weight restrictions or outcomes is unclear.