When describing growth, a fetus may be appropriate for gestational age (AGA), too small (SGA, sometimes confused with intra uterine growth restriction, IUGR) or too large (LGA), a condition often defined as macrosomia. While the literature on IUGR is extensive and many lectures/conferences are dedicated to this topic, macrosomia is less often discussed.
Macrosomia is a term that should be applied only to the newborn, since it relates to an actual weight. When describing a fetus, the only term one should use is LGA. There are various definitions for macrosomia. In developed countries, the most commonly used threshold is weight above 4500 g (9 lb 15 oz). Weight above 4000 g (8 lb 13 oz) or 10 lb (4536 g) are also commonly used. When describing fetuses, weight percentiles for gestational age, rather than actual estimated weight, are preferable.
Etiologies for macrosomia include metabolic (maternal diabetes), genetic (various overgrowth syndromes such as Beckwith-Wiedemann), constitutional as well as maternal factors, such as obesity and excessive gestational weight gain.
There are maternal and fetal risk factors: for example, increased incidence of cesarean deliveries and genital tract lacerations as well as shoulder dystocia.
During pregnancy, ultrasound is the ideal diagnostic tool, although its precision declines as fetal weight increases. Sonographic measurements of the fetal head, abdomen and femur are the most commonly employed criteria but soft tissues and volumes (obtained, generally, by 3D methods) have also been described. Measuring the fetal abdominal circumference (AC) may be the best tool for diagnosing risk of macrosomia, with AC >35 cm identifying >90% of fetuses with macrosomia that are at risk for shoulder dystocia.