The Committee on Practice Bulletins—Obstetrics of the American College of Obstetricians and Gynecologists (ACOG) has issued new clinical management guidelines on fetal macrosomia. ACOG Practice Bulletin No. 22, which replaces Technical Bulletin No. 159 issued in September 1991, appears in the November 2000 issue of Obstetrics and Gynecology. These guidelines discuss risk factors and complications, and suggest clinical management for the pregnancy with suspected fetal macrosomia.

Diagnosis, Risk Factors and Complications

The term fetal macrosomia implies fetal growth beyond a specific weight, usually 4,000 g (8 lb, 13 oz) or 4,500 g (9 lb, 4 oz), regardless of the fetal gestational age. Results from large cohort studies support the use of 4,500 g as the weight at which a fetus should be considered macrosomic.

Weighing the newborn after delivery is the only way to accurately diagnose macrosomia, because the prenatal diagnostic methods (assessment of maternal risk factors, clinical examination and ultrasonographic measurement of the fetus) remain imprecise. Leopold's maneuvers and measurement of the height of the uterine fundus above the maternal symphysis pubis are the two primary methods for the clinical estimation of fetal weight, according to ACOG. Use of either of these methods alone is considered to be a poor predictor of fetal macrosomia; therefore, they must be combined to produce a more accurate measurement. Ultrasonographic measurement of the fetus serves as a means to rule out the diagnosis of fetal macrosomia, which may aid in avoiding maternal morbidity, but is considered to be no more accurate than Leopold's maneuver.

According to the ACOG committee, the risk factors (excluding preexisting diabetes mellitus) for fetal macrosomia, in decreasing order of importance, are as follows: a history of macrosomia, maternal prepregnancy weight, weight gain during pregnancy, multiparity, male fetus, gestational age more than 40 weeks, ethnicity, maternal birth weight, maternal height, maternal age younger than 17 years and a positive 50-g glucose screen with a negative result on the three-hour glucose tolerance test.

Pregestational diabetes and gestational diabetes are also associated with fetal macrosomia. Data from one study demonstrated that women with untreated borderline gestational diabetes had an increased risk of delivering infants weighing more than 4,500 g, compared with women who had normal glucose tolerance levels (6 percent versus 2 percent, respectively). If gestational diabetes remains undiagnosed and untreated, the risk of macrosomia may be as high as 20 percent.

ACOG emphasizes that an increased risk of cesarean delivery is the primary maternal risk factor associated with macrosomia. Results from cohort studies demonstrate that the risk of cesarean delivery in women attempting a vaginal delivery at least doubles when the fetal weight is estimated to be more than 4,500 g.

Although rare (complicating 1.4 percent of all vaginal deliveries), shoulder dystocia is the most serious complication associated with fetal macrosomia. When birth weight is more than 4,500 g, however, the risk is increased to 9.2 to 24 percent in pregnant women without diabetes and to 19.9 to 50 percent in pregnancies complicated by diabetes. However, while macrosomia increases risk, shoulder dystocia also occurs unpredictably in infants of normal birth weight.

Fracture of the clavicle and damage to the nerves of the brachial plexus are the most common fetal injuries associated with macrosomia. In macrosomic infants, the risk of clavicular fracture and brachial plexus injury is approximately 10-fold and 18- to 21-fold, respectively, when birth weight is more than 4,500 g.

Clinical Considerations

The ACOG practice bulletin discusses the following clinical considerations:

Clinical Intervention.

Clinical interventions for the treatment of suspected macrosomia (in pregnant women without diabetes) have not been reported. In pregnancies complicated by diabetes, one small clinical trial evaluated the effect of dietary intervention with or without the addition of insulin. Results s uggest that the addition of insulin might be of benefit in treating early macrosomia (between 29 and 33 weeks of gestation). The data revealed a decreased likelihood of birth weight greater than the 90th percentile from 45 percent among the study participants treated with diet only to 13 percent among those receiving insulin in addition to dietary intervention.

Excessive weight gain during pregnancy is associated with fetal macrosomia, and results from large cohort studies confirm this. However, no data are available on the role of dietary restrictions during pregnancy to prevent macrosomia in obese women who do not have diabetes.

Cesarean Delivery.

The role of cesarean delivery in suspected fetal macrosomia remains controversial. While the risk of birth trauma with vaginal delivery is higher with increased birth weight, cesarean delivery reduces, but does not eliminate, this risk. In addition, randomized clinical trial results have not shown the clinical effectiveness of prophylactic cesarean delivery when any specific estimated fetal weight is unknown. Results from large cohort and case-control studies reveal that it is safe to allow a trial of labor for estimated fetal weight of more than 4,000 g. Nonetheless, the results of these reports, along with published cost-effectiveness data, do not support prophylactic cesarean delivery for suspected fetal macrosomia with estimated weights of less than 5,000 g (11 lb), although some authors agree that cesarean delivery in these situations should be considered.

Induction of Labor.

In cases of term patients with suspected fetal macrosomia, current evidence does not support early induction of labor. Results from recent reports indicate that induction of labor at least doubles the risk of cesarean delivery without reducing the risk of shoulder dystocia or newborn morbidity, although the results are affected by small sample size and bias caused by the retrospective nature of the reports. Results from one randomized clinical trial reveal similar cesarean delivery rates in the induction group (19.4 percent) compared with the expectant management group (21.6 percent), with five cases of shoulder dystocia in the induction group and six cases in the expectant management group.

Suspected Fetal Macrosomia and the Management of Labor and Vaginal Delivery.

Midpelvic operative vaginal delivery is the most important consideration for labor and delivery in the case of suspected fetal macrosomia. With the exception of extreme emergencies, a cesarean delivery should be performed for midpelvic arrest of the fetus with suspected macrosomia. If a decision is made to perform a cesarean delivery in the presence of suspected macrosomia, the incision should be large enough to avoid a difficult abdominal delivery.

Summary of Recommendations

The ACOG committee provides the following recommendations for the management of fetal macrosomia:

Recommendations based on good and consistent scientific evidence (Level A):

• The diagnosis of fetal macrosomia is imprecise. For suspected fetal macrosomia, the accuracy of estimated fetal weight using ultrasound biometry is no better than that obtained with clinical palpation (Leopold's maneuvers).

Recommendations based on limited or inconsistent scientific evidence (Level B):

• Suspected fetal macrosomia is not an indication for induction of labor, because induction does not improve maternal or fetal outcomes.

• Labor and vaginal delivery are not contraindicated for women with estimated fetal weights up to 5,000 g in the absence of maternal diabetes.

• With an estimated fetal weight more than 4,500 g, a prolonged second stage of labor or arrest of descent in the second stage is an indication for cesarean delivery.

Recommendations based primarily on consensus and expert opinion (Level C):

• Although the diagnosis of fetal macrosomia is imprecise, prophylactic cesarean delivery may be considered for suspected fetal macrosomia with estimated fetal weights of more than 5,000 g in pregnant women without diabetes and more than 4,500 g in pregnant women with diabetes.

• Suspected fetal macrosomia is not a contraindication to attempted vaginal birth after a previous cesarean delivery.