The term fetal macrosomia implies growth beyond an absolute birth weight of 4000 grams or 4500 grams, regardless of gestational age. The risk of morbidity for both infants and mothers increases when the birthweight is between 4000 and 4500 grams. Risks for maternal and newborn morbidity rise considerably with birthweights >4500g. A correct diagnosis can only be made after weighing an infant at birth, as ultrasound prediction is not precise.
In the United States, 7.8% of all live-born infants weigh > 4000 grams and 1% weigh > 4500 grams. The most serious complication of fetal macrosomia is shoulder dystocia which complicates 0.2-3.0% of all vaginal deliveries. When the birthweight is > 4500 grams, the shoulder dystocia rate increases to 9-14%. The shoulder dystocia rate increases to 20-50% in the presence of maternal diabetes when the birthweight is > 4500 grams. ‘Large for gestational age (LGA)’ also refers to excessive fetal growth, but rather than absolute weight, LGA is usually defined as ≥90th percentile for a given gestational age.
Note: Upon review of current literature, ACOG states
No single formula based on ultrasound biometry performs significantly better than others for the detection of macrosomia more than 4,500 g
Similar to clinical estimates of fetal weight, ultrasonography can be used most effectively as a tool to rule out macrosomia, which may help avoid maternal and fetal morbidity
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