Fetal macrosomia is distinct from the term “large for gestational age” (“LGA”). LGA is defined as fetal weight above the 90th percentile for the baby’s gestational age, based upon growth curves generated from national statistics. In contrast, fetal macrosomia describes a baby weighing more than 4,000 g (about 8 lbs., 12.75 oz.), or more than 4,500 g (about 9 lbs. 14.5 oz.), regardless of gestational age. Thus, a baby weighing 3,900 g at 38 weeks gestational age could be LGA (the 90th percentile at that age is about 3,867 g), but fetal macrosomia would not be present.
Studies have shown that the risks of injury for infants and mothers increase when birth weight is greater than 4,000 g, and those risks increase even more sharply when birth weight exceeds 4,500 g. While many health care providers disagree as to the precise weight in grams which would cause a baby to be characterized as “macrosomic,” most reasonable care providers use either 4,000 grams or 4,500 grams as the boundary for the diagnosis of fetal macrosomia.
A diagnosis of fetal macrosomia is not rare. Statistically, about 10% of life infants in the United States weigh more than 4,000 g, and approximately 1.5% weight more than 4,500 g. Maternal diabetes and maternal obesity are the most common factors leading to fetal macrosomia. Other risk factors include post-term or post-dates pregnancy, multiparity, and excessive maternal weight gain.
During pregnancy, a safe and careful obstetrician will typically assess the prospects for the development of fetal macrosomia in light of maternal risk factors, clinical measurements and ultrasonographic imaging technologies. Although prenatal estimates of fetal weight may not be exact, evidence of fetal macrosomia as confirmed by prenatal fetal and maternal assessments will not be as safely ignored by the safe and careful obstetrician. The risks of an injury and death associated with fetal macrosomia are well recognized.
Because of the grave risks associated with fetal macrosomia, many safe and careful obstetricians will deliver their patients prior to term. Studies demonstrate that the risk of cesarean delivery doubles when birth weight exceeds 4,500 g. primarily due to abnormalities in labor. Moreover, the risks to the baby of nerve injury and brachial plexus palsy increase markedly with fetal macrosomia. Even more gravely, the risks of depressed Apgar scores and admission to neonatal intensive care units are increased in the presence of fetal macrosomia.