eMedicine Specialties > Obstetrics and Gynecology > Obstetrical Complications
Macrosomia
Updated: Dec 3, 2007
Introduction
Background
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.1
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.
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.2 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.3
Pathophysiology
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 cervical4 , 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.5
Frequency
United States
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).
International
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.
Mortality/Morbidity
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.5
- Maternal: 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 al6 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.
- Neonatal: 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.4 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%).
- Fetal: 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.5
Race
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.
Sex
Male infants are more likely to be macrosomic than female infants.
Age
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.
Clinical
History
Fetal macrosomia has been defined in several ways. The definitions include birth weight greater than 4000-4500 g or greater than 90% for the newborn adjusted for race, sex, and gestational age. Based on these definitions, macrosomia occurs in 1-10% of all deliveries. Macrosomia may place the mother and fetus or neonate at risk for adverse outcomes. Antenatal risk factors reportedly predict macrosomia at birth. Identification of these at-risk pregnancies may allow intervention to reduce the risk, to provide appropriate counseling, and to implement appropriate plans for monitoring and follow-up care during pregnancy and after delivery.
- Maternal diabetes is one of the strongest risk factors associated with giving birth to an infant that is considered large for gestational age. Pregestational and gestational diabetes result in fetal macrosomia in as many as 50% of pregnancies complicated by gestational diabetes and in 40% of those complicated by type 1 diabetes mellitus. Studies of macrosomic infants of diabetic mothers reveal a greater amount of total body fat, thicker upper-extremity skin fold measurements, and smaller ratios of head to abdominal circumference than macrosomic infants of nondiabetic mothers.3
- Maternal weight prior to pregnancy can affect the weight of the fetus. Women who are obese are more likely to have larger infants.7
- Excessive weight gain in pregnancy is a risk factor for macrosomia. The risk is greater for women with obesity than for women without obesity.7
- Gestational age is associated with macrosomia. Birth weight increases as gestational age increases. Prolonged pregnancies (>41 wk) are associated with an increased incidence of macrosomia. Macrosomic infants account for about 1% of term deliveries and 3-10% of postterm deliveries.3
- Multiparity and grand multiparity increase the risk of macrosomia.8 Parity has been reported to be associated with 100-150 grams of weight gain at birth.9
- A history of macrosomia can influence future pregnancies. Women who previously delivered a macrosomic fetus are 5-10 times more likely than women without such a history to deliver a baby considered large for gestational age the next time they become pregnant.
- Fetal sex influences macrosomic potential. Male infants weigh more than female infants at any gestational age. Recent studies have confirmed this association.10
- Excessive amniotic fluid defined as greater than or equal to 60th percentile for gestational age has recently been associated with macrosomia.11
- Despite these so-called risk factors for macrosomia, much of the variation in birth weights remains unexplained. Most infants who weigh more than 4500 g have no identifiable risk factors.
Physical
Many physical examination findings help identify a pregnancy at risk for macrosomia.
- Maternal obesity is associated with fetal macrosomia. Maternal body mass index (BMI) is a good way of diagnosing obesity prior to pregnancy. Calculated based on height in meters and weight in kilograms, BMI is determined using the following formula:
BMI = (weight [kg])/(height [m])2- Body fat percentage can be estimated using the following Deurenberg equation:
Body fat percentage = 1.2(BMI) + 0.23(age [y]) – 10.8(sex) – 5.4
Males are coded as 1 and females are coded as 0. This formula has a standard error of 4% and explains approximately 80% of the variation in body fat. - The most widely accepted definition for obesity is that based on the World Health Organization criteria, which uses the BMI. Under this convention for adults, grade 1 overweight (commonly and simply called overweight) is a BMI of 25-29.9 kg/m2. Grade 2 overweight (commonly called obesity) is a BMI of 30-39.9 kg/m2. Grade 3 overweight (commonly called severe or morbid obesity) is a BMI greater than or equal to 40 kg/m2.
- A BMI greater than 30 kg/m2 is associated with larger infants at delivery.
- Body fat percentage can be estimated using the following Deurenberg equation:
- The recommendations for weight gain in pregnancy have been based on the Institute of Medicine (IOM) guidelines published in 1990. The suggested weight gain is 11.2–15.9 kg (25–35 lb) for women with a normal BMI, 6.8 –11.2 kg (15–25 lb) for women who are overweight, and 6.8 kg (15 lb) for women who are obese.12 Excessive weight gain in pregnancy, especially in women who are already obese, is a risk factor for macrosomia.
- Fundal height measurements are an inaccurate way of estimating fetal size. They are influenced by maternal size, the amount of amniotic fluid, the status of the bladder, the presence of pelvic masses (eg, fibroids), fetal position, and many other factors. However, in general, a fundal height that is 3-4 cm larger than the gestational age of the pregnancy in the third trimester necessitates further testing to determine the cause. Excessive fetal growth can be one reason for the discrepancy between the fundal height and the gestational age of the pregnancy.
- Leopold maneuvers are techniques developed to determine fetal presentation, lie, and size. They are also limited by many factors, as mentioned previously for fundal height measurements. However, these maneuvers provide the clinician with a general appreciation of fetal size and other important information. Prospective studies designed to evaluate Leopold maneuvers with fundal height measurement for the prenatal diagnosis of possible macrosomia report sensitivities of 10-43%, specificities of 99-99.8%, and positive predictive values of 28-53%.13,14
Causes
Causes for macrosomia include factors that contribute to excessive fetal growth and weight gain.
- Diabetes that is poorly controlled in pregnancy is the greatest risk factor for fetal macrosomia. This is believed to be partially explained by excessive growth due to elevated maternal plasma glucose levels and resulting elevated insulin and insulinlike growth factor levels, which stimulate glycogen synthesis, fat deposition, and fetal growth.
- Excessive maternal weight gain and/or prepregnancy weight also play the some role in macrosomia by providing excessive growth in selected cases. Whether this is also due to undiagnosed glucose intolerance in these individuals remains to be studied.
- Genetic factors also contribute to fetal size. Taller and heavier parents typically produce larger offspring.
More on Macrosomia |
 Overview: Macrosomia |
| Differential Diagnoses & Workup: Macrosomia |
| Treatment & Medication: Macrosomia |
| Follow-up: Macrosomia |
| References |
| Next Page » |
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Further Reading
Keywords
megasomia, macrosomic neonate, macrosomic newborn, large for gestational age, LGA, body mass index, BMI, maternal diabetes, maternal obesity, maternal weight gain, hyperglycemia, shoulder dystocia, birth trauma, birth canal laceration, neonatal morbidity, neonatal injury, maternal injury, cesarean delivery, cesarean section, caesarean delivery, caesarean section, cesarean birth, caesarean birth, advanced gestational age, perineal laceration, vaginal laceration, cervical laceration, still birth, stillborn, multiparity, multi-parity, grand multiparity, diabetic pregnancy, poorly controlled diabetes, diabetes, diabetes mellitus, gestational diabetes, macrosomic pregnancy, excessive maternal weight gain, undiagnosed glucose intolerance
