Fetal macrosomia

Definition

• Fetal macrosomia denotes excessive fetal growth, defined by an absolute birth weight irrespective of gestational age; thresholds of 4000 g or 4500 g are used, and there is no universal definition.
• It is distinguished from LGA (large for gestational age), a birth weight at or above the 90th centile for gestational age.

Classification

• Risks rise with weight: between 4000 and 4499 g complications increase moderately, above 4500 g steeply, and they are highest above 5000 g.

Occurrence & epidemiology

Epidemiology

• In a meta-analysis, 15.8% of obese mothers but only 9.3% of underweight or normal-weight mothers gave birth to a baby weighing at least 4000 g.

Aetiopathogenesis

Aetiology and risk factors

• Apart from genetically determined size, maternal diabetes is the most important cause: maternal hyperglycaemia leads to high fetal insulin levels, which have anabolic effects; the poorer the metabolic control, the larger the fetus.
• Maternal obesity also contributes; in the meta-analysis mentioned, the unadjusted odds ratio for a birth weight of 4000 g or more was 2.17.
• Rare causes are syndromes such as Beckwith-Wiedemann, Sotos, Marshall and Weaver syndromes.

Clinical features

Clinical features and complications

• Birth may be difficult; typical birth injuries are shoulder dystocia, fractures of the clavicle or limbs and perinatal asphyxia, and brachial plexus injury and stillbirth rise steeply above 4500 g.
• Neonates more often have respiratory distress, meconium aspiration, hypoglycaemia and polycythaemia; infants of diabetic mothers are additionally at risk of hypocalcaemia, hypomagnesaemia, hyperbilirubinaemia, respiratory distress syndrome and certain congenital anomalies.
• The neonates appear large and plethoric, and the 5-minute Apgar score may be low.

Diagnosis

• Antenatal prediction is difficult; birth weight is decisive.
• Sonographic weight estimation becomes less accurate as weight increases; for birth weights above 4500 g its sensitivity is only 33–44%.
• Abdominal palpation is also unreliable, and maternal obesity further hampers assessment.

Keep learning in the app

Further reading (selection)

1. StatPearls: Macrosomia (NCBI Bookshelf)
2. MSD Manual Professional Edition: Large-for-Gestational-Age (LGA) Infant
3. Biomed Res Int 2014: Maternal obesity and occurrence of fetal macrosomia – systematic review and meta-analysis (PMC, open access)

Cross-references