1. What every clinician should know
Clinical features and incidence
Macrosomia is a term used to describe excessive fetal growth. No threshold weight has been universally accepted, but common definitions include a fetal weight above 4,000 or 4,500 grams. The frequency of macrosomia among liveborn infants varies not only with the definition used, but also by population and maternal characteristics. In the United States, approximately 10% of infants weigh more than 4000 grams at birth whereas 1% of infants weigh more than 4500 grams.
Multiple risk factors for macrosomia have been identified. Maternal risk factors include obesity, excessive weight gain, previous macrosomic infant, advanced maternal age, multiparity, Hispanic or African American ethnicity, and either diabetes (pregestational or gestational) or impaired glucose tolerance. Fetal risk factors include male gender, prolonged pregnancy (beyond 42 weeks), and genetic abnormalities such as Beckwith-Wiedemann syndrome. Despite the identification of multiple risk factors, none can be used to accurately predict excessive fetal growth.
2. Diagnosis and differential diagnosis
Establishing the diagnosis
Fetal weight can be estimated by physical exam or sonographically.
Although symphysis-fundus height measurements can be used to assess fetal growth, clinical estimates of fetal weight are generally made using Leopold maneuvers in which the fetus is palpated through the maternal abdomen. Advantages of Leopold maneuvers include the fact that the procedure is relatively easy to perform and does not incur the expense of imaging studies, but disadvantages include low sensitivity (below 50%) and positive predictive value (below 50%) for macrosomia.
In general, clinical estimates of fetal weight are more likely to underestimate the weight of macrosomic infants than to overestimate the weight. Of note, parous women have been shown to be as likely to predict a fetal weight greater than 4000 grams as clinicians using Leopold maneuvers to detect macrosomia.
2D ultrasound remains the most common imaging test used to assess fetal growth and to estimate fetal weight. An abdominal circumference measuring more than 35-38 cm is the most reliable single parameter used to assess risk of macrosomia. More commonly, however, multiple biometric parameters (including head circumference, abdominal circumference and femur length) are entered into a formula to calculate a composite estimated fetal weight.
Because 2D estimates of fetal weight do not account for variation in fetal body composition and because of the margin of error inherent to sonographic estimates of fetal weight, the accuracy of 2D ultrasound has varied widely across publications. The margin of error associated with ultrasound estimates of fetal weight above 4,500 grams are higher than the margin of error associated with ultrasound estimates of fetal weight for non-macrosomic infants (13% vs 8% respectively).
3D ultrasound, which can be used to assess fetal body composition and to perform measurements of fetal subcutaneous fat, has emerged as a promising tool. While not yet the standard of care, 3D ultrasound may be used in combination with 2D ultrasound to generate more accurate sonographic estimates of fetal weight in the future.
Clinical estimates of fetal weight may be affected by maternal habitus and fetal position. In addition, clinical assessments suggestive of excessive fetal growth also may be caused by an increase in amniotic fluid volume, multiple gestation or fetal hydrops. When clinical estimates of fetal weight are suggestive of macrosomia, sonographic evaluation is recommended to exclude other conditions that may affect clinical estimates of fetal size.
Sonographic estimates of fetal weight may be limited in the setting of maternal obesity, an engaged fetal vertex late in gestation and oligohydramnios. In addition, ultrasound tends to overestimate fetal size in diabetic pregnancies. Specifically, because fetuses of diabetic pregnancies tend to have relatively large abdominal circumference measurements, calculated estimates of fetal weight in such pregnancies tend to be higher than in non-diabetic pregnancies; however, because the increased abdominal circumference in these fetuses is due to the deposition of adipose tissue which has a lower density relative to other fetal tissues, the estimated fetal weight calculated using these abdominal circumference measurements may lead to an overestimation of fetal weight in diabetic pregnancies.
When clinical estimates of fetal weight are suggestive of excessive growth, sonographic evaluation is recommended to assess fetal growth and to exclude other conditions that may affect clinical estimates of fetal size. If a sonographic estimate of fetal weight is suggestive of excessive growth, results should be correlated with a patient’s diabetic status; for diabetic patients, a diagnosis of excessive fetal growth should prompt a review of glycemic control.
Although one study demonstrated that initiation of insulin therapy in a diet-controlled diabetic gestation led to a reduction in the likelihood of an infant birthweight above the 90th percentile, this practice is not commonly utilized. No intervention is recommended for non-diabetic women to restrict fetal growth. When excessive fetal growth is noted in the preterm period, repeat sonographic assessment should be considered later in gestation so a more current estimated fetal weight is available for delivery planning.
Macrosomia is associated with an increased risk of maternal and neonatal morbidity. The American College of Obstetricians and Gynecologists (ACOG), states that “a prolonged second stage of labor or arrest of descent in the second stage is an indication for cesarean delivery” and that “prophylactic cesarean delivery may be considered … with estimated fetal weights greater than 5,000 g in women without diabetes and greater than 4,500 g in women with diabetes.”
However, ACOG states that “suspected fetal macrosomia is not an indication for induction of labor because induction does not improve maternal or fetal outcomes.” Because operative vaginal delivery of macrosomic infants is associated with an increased risk of shoulder dystocia, cesarean delivery is recommended over operative vaginal delivery for arrest of descent in cases of suspected macrosomia.
In cases of shoulder dystocia or difficult delivery, infants should be assessed for birth injury regardless of birthweight. For infants noted to have excessive fetal growth (either large for gestational age or macrosomia), postnatal evaluation is recommended to determine the etiology. In addition, macrosomic infants should be evaluated for hypoglycemia and/or polycythemia.
There is an increased risk of morbidity, both maternal and neonatal, with birthweights above 4,000 grams. Maternal risks include protraction and arrest disorders in labor, operative vaginal delivery, cesarean delivery, lacerations and postpartum hemorrhage. Importantly, among infants with a birth weight above 4,500 grams, shoulder dystocia occurs in approximately 10-20% of vaginal deliveries of non-diabetic women and 20-50% of diabetic women.
While neonatal risks are increased in the setting of shoulder dystocia, macrosomia is associated with an increased risk of birth trauma even in the absence of shoulder dystocia. Examples of neonatal injuries associated with macrosomia include brachial plexus injury, clavicular fractures, soft-tissue bruising, cephalohematoma, subgaleal hemorrhage and diaphragm paralysis.
Postnatally, macrosomic infants are at risk of hypoglycemia and polycythemia secondary to residual hyperinsulinemia resulting from increased intrauterine glucose exposure. If excessive fetal growth is diagnosed prenatally, antepartum maternal weight gain should be reviewed and nutritional counseling considered in cases of excessive weight gain. In diabetic pregnancies complicated by excessive fetal growth, every effort should be made to ensure adequate glycemic control in the antepartum period.
5. Prognosis and outcome
Patients diagnosed with excessive fetal growth should be counseled about the risk of intrapartum, postpartum and neonatal complications related to macrosomia. Published literature also suggests that there is a higher frequency of minor congenital anomalies and perinatal death among macrosomic infants than among infants with birth weights below 4,000 grams and appropriate for gestational age. In addition to the immediate perinatal complications associated with excessive fetal growth, individuals who were macrosomic at birth are at risk for additional long term sequelae including obesity, impaired glucose tolerance and metabolic syndrome later in life.
6. What is the evidence for specific management and treatment recommendations
“ACOG Practice Bulletin Number 22”. Fetal Macrosomia. November 2000. (This invaluable document provides evidence-based management guidelines for antepartum and intrapartum care.)
Alexander, GR, Himes, JH, Kaufman, RB, Mor, J, Kogan, M. “A United States national reference for fetal growth”. Obstet Gynecol. vol. 87. 1996. pp. 163-8. (This document presents the 50th, 90th and 95th percentiles for birth weight using data on 3.8 million births within the 1991 U.S. Live Birth File of the National Center for Health Statistics.)
Alsulyman, OM, Ouzounian, JG, Kjos, SL. “The accuracy of intrapartum ultrasonographic fetal weight estimation in diabetic pregnancies”. Am J Obstet Gynecol. vol. 177. 1997. pp. 503-6. (This large study compares the accuracy of ultrasound estimates of fetal weight in diabetic vs. non-diabetic pregnancies.)
Chauhan, SP, Cowan, BD, Magann, EF, Bradford, TH, Roberts, WE. “Intrapartum detection of a macrosomic fetus: clinical versus 8 sonographic models”. Aust N Z J Obstet Gynaecol. vol. 35. 1995. pp. 266-70. (This prospective study compares the accuracy of clinical vs. sonographic methods in detecting macrosomia.)
Nesbitt, TS, Gilbert, WM, Herrchen, B. “Shoulder dystocia and associated risk factors with macrosomic infants born in California”. Am J Obstet Gynecol. vol. 179. 1998. pp. 476-80. (This large cohort study shows the relationship between birth weight, diabetes and mode of delivery on the rate of shoulder dystocia.)
Okun, N, Verma, A, Mitchell, BF, Flowerdew, G. “Relative importance of maternal constitutional factors and glucose intolerance of pregnancy in the development of newborn macrosomia”. J Matern Fetal Med. vol. 6. 1997. pp. 285-90. (This large study assesses the strength of a number of risk factors for macrosomia.)
Sacks, DA, Chen, W. “Estimating fetal weight in the management of macrosomia”. Obstet Gynecol Surv. vol. 55. 2000. pp. 229-39. (A review summarizing the issues related to prenatal estimation of fetal weight and the impact on shoulder dystocia frequency.)
Sherman, DJ, Arieli, S, Tovbin, J, Siegel, G, Caspi, E. “A comparison of clinical and ultrasonic estimation of fetal weight”. Obstet Gynecol. vol. 91. 1998. pp. 212-7. (This large study evaluates the accuracy of ultrasound estimates of fetal weight at different strata of fetal size.)
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- 1. What every clinician should know
- 2. Diagnosis and differential diagnosis
- 3. Management
- 4. Complications
- 5. Prognosis and outcome
- 6. What is the evidence for specific management and treatment recommendations