Fetal Macrosomia

Definition

While the terms large for gestational age (LGA) and fetal macrosomia are both meant to convey a concern for excessive growth, the two terms differ slightly in their specific definitions. LGA refers to neonatal birth weight larger than the 90th percentile for a given gestational age. In contrast to LGA, fetal macrosomia is defined as an absolute birth weight above a specified threshold regardless of gestational age. Varying thresholds have been used, including 4000 g and 4500 g, and both thresholds have been associated with adverse pregnancy outcomes. The American College of Obstetricians and Gynecologists suggests a threshold of 4500 g for considering changes in management, secondary to estimated fetal weight for women with diabetes, and a threshold of 5000 g for women without diabetes.

Prevalence and Epidemiology

The prevalence of macrosomia depends on the definition used and the population studied. In the United States in 2014, 6.9% of neonates weighed more than 4000 g, 1% weighed more than 4500 g, and about 0.1% of neonates weighed more than 5000 g.

Etiology, Pathophysiology, and Embryology

Fetal macrosomia is a description of excessive fetal size. In many cases, it represents a consequence of a multitude of differing environmental and genetic factors that ultimately result in the macrosomic state. Multiple risk factors contribute to fetal macrosomia, some of which are discussed subsequently. Risk factors include:

• 1.

  Maternal obesity, defined as a prepregnancy body mass index greater than 30, and excessive gestational weight gain are two important risk factors for macrosomia given the high prevalence of both in the United States.

• 2.

  Maternal diabetes and gestational diabetes are well-recognized risk factors for macrosomia because chronic and intermittent maternal hyperglycemia has been strongly associated with accelerated fetal growth and fat deposition. Abnormal glucose tolerance without the diagnosis of gestational diabetes has even been suggested to increase the risk of macrosomia. Management of gestational diabetes with glucose monitoring, nutritional consultation, and hypoglycemic agents, when indicated, significantly mitigates this risk.

• 3.

  Postterm pregnancy is a risk factor. In the United States, after 42 weeks' gestation, 25.3% of fetuses weigh more than 4000 g, and 5.2% weigh more than 4500 g.

• 4.

  Genetic syndromes are rare in fetuses with macrosomia without other findings but must be considered, especially when other congenital abnormalities are identified (see Differential Diagnosis From Imaging Findings ).

• 5.

  Other important risk factors are multiparity, prior macrosomic infants, male sex, ethnicity, maternal birth weight greater than 4000 g, and advanced maternal age.

In some infants, the pathophysiology is clear, such as postterm pregnancy or poorly controlled diabetes. However, the etiology of macrosomia is unclear in many cases. More recently, it has become clear that increases in maternal glycemia, even at levels thought to rule out diabetes, are associated with increasing fetal size and LGA. Many of the aforementioned listed risk factors may ultimately be related to subclinical maternal glucose intolerance, including diabetes, obesity, excessive gestational weight gain, maternal birth weight, and advancing age.