- Author: Allahyar Jazayeri, MD, PhD, FACOG, DACOG, FSMFM; Chief Editor: Christine Isaacs, MD more...
The term macrosomia is used to describe a newborn with an excessive birth weight. A diagnosis of fetal macrosomia can be made only by measuring birth weight after delivery; therefore, the condition is confirmed only retrospectively, ie, after delivery of the neonate. Fetal macrosomia is encountered in up to 10% of deliveries.
Attempts at perinatal diagnosis of macrosomia have proven difficult and are often inaccurate. This article defines macrosomia and reviews clinical and diagnostic modalities currently used to screen for pregnancies at the greatest risk for macrosomia with some degree of accuracy. Maternal, fetal, and neonatal consequences of macrosomia are also discussed, with specific attention to the potential etiology of macrosomia.
Fetal macrosomia has been defined in several different ways, including birth weight of 4000-4500 g (8 lb 13 oz to 9 lb 15 oz) or greater than 90% for gestational age after correcting for neonatal sex and ethnicity. Based on these definitions, macrosomia affects 1-10% of all pregnancies.
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Factors associated with fetal macrosomia include genetics; duration of gestation; presence of gestational diabetes; and class A, B, and C diabetes mellitus. Genetic, racial, and ethnic factors influence birth weight and the risk of macrosomia. Male newborns typically weigh more than female newborns and thus comprise a greater proportion of infants with birth weights exceeding 4500 g at any gestational age. The risk of macrosomia also varies with ethnicity. Even when controlled for diabetes, studies have demonstrated that Hispanic women have a higher risk of fetal macrosomia compared with white, African American, or Asian women. Genetic factors, such as parental height and weight, may also play a role in determining newborn birth weight.
Despite the identification and characterization of risk factors, no combination of these risk factors can predict macrosomia accurately enough to be used clinically. Much of the birth weight variation remains unexplained, and most macrosomic infants do not have identifiable risk factors. Finally, macrosomia is reportedly associated with neonatal morbidity, neonatal injury, maternal injury, and cesarean delivery.
The pathophysiology of macrosomia is related to the associated maternal or fetal condition that accounts for its development. In general, poorly controlled diabetes, maternal obesity, and excessive maternal weight gain are all associated with macrosomia and have intermittent periods of hyperglycemia in common. Hyperglycemia in the fetus results in the stimulation of insulin, insulinlike growth factors, growth hormone, and other growth factors, which, in turn, stimulate fetal growth and deposition of fat and glycogen. Advanced gestational age results in a larger birth weight at delivery by allowing the growth process to continue in utero.
Macrosomia may be associated with birth trauma for the neonate and birth canal lacerations, eg, perineal, vaginal, and cervical , or cesarean delivery for the mother. However, macrosomia in the neonate of a diabetic mother may indicate poor glucose control. These infants are at increased risk of intrauterine death and thus require close monitoring and antepartum fetal testing.
Infants with a birth weight of 4000 g (8 lb 13 oz) or more comprise up to 10% of infants born in the United States, and, in 1998, 1.5% of all neonates had a birth weight equal to or greater than 4500 g (9 lb 15 oz).
Variation in the percentage of macrosomia in different ethnic groups has been observed independent of diabetes. In general, Hispanic women have a larger proportion of macrosomic newborns compared with white, African American, or Asian women.
Morbidity and mortality associated with macrosomia can be divided into maternal, fetal, and neonatal categories. A recent study investigating the effects of birth weight on fetal mortality shows that higher fetal mortality rates are associated with a birth weight of greater than 4250 g in nondiabetic mothers and a birth weight of 4000 g in diabetic mothers.
Macrosomia is associated with a higher incidence of cesarean delivery (double that of control subjects) and with birth canal lacerations associated with vaginal delivery. Mulik et al reviewed the outcomes of 8617 deliveries over a period of 11 years. In that population, 666 neonates were born with a birth weight of 4000-4499 g and 97 neonates were larger than 4500 g. In their study, Mulik et al found maternal morbidity to be associated with a birth weight of 4500 g or higher compared with a birth weight of less than 4000 g. Postpartum hemorrhage occurred in 3.1% of mothers with newborns weighing 4500 g or more compared with 1.5% in mothers with newborns weighing less than 4000 g. Blood transfusions occurred in 15.4% of mothers with newborns weighing 4500 g or more compared with 3.1% in mothers with newborns weighing less than 4000 g.
Macrosomic neonates are at risk for shoulder dystocia and birth trauma. This risk is directly related to neonatal birth weight and begins to increase substantially when birth weight exceeds 4500 g and particularly when it exceeds 5000 g. Brachial plexus injury is rare, with an incidence of fewer than 2 cases per 1000 vaginal deliveries. This risk is approximately 20 times higher when the birth weight is more than 4500 g. Mulik et al reported a higher incidence of NICU admissions for neonates with a birth weight higher than 4500 g compared with newborns with a birth weight of less than 4000 g (9.3% vs 2.7%). Risk of shoulder dystocia was 10 times higher in the larger babies (4.1% vs 0.4%).
When associated with diabetes, fetal macrosomia indicates poor maternal glucose control, and these infants are at risk of stillbirth. Stillbirth rates in macrosomic infants are twice as high as those in control subjects, irrespective of diabetes. However, for a birth weight of 4500-5000 g, the fetal death rate is fewer than 2 deaths per 1000 births for nondiabetic women and is approximately 8 deaths per 1000 births for diabetic women. For a birth weight of 5000-5500 g, this rate is 5-18 deaths per 1000 births for nondiabetic women and is approximately 40 deaths per 1000 births for diabetic women.
Macrosomia occurs with higher frequency in newborns of Hispanic origin. Because Hispanic women have a higher incidence of diabetes during pregnancy, part of the preponderance of macrosomia in this ethnic group is due to the higher incidence of diabetes in pregnancy. However, even when corrected for diabetes, Hispanic mothers tend to have larger newborns.
Male infants are more likely to be macrosomic than female infants.
Macrosomia, as defined by birth weight greater than 4000-4500 g, occurs with higher frequency in prolonged pregnancies that continue beyond the expected delivery date.
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