Two terms have been used to define excessive fetal growth. Fetal macrosomia is based on weight alone and refers to afetus with an estimated weight of 4000–4500 g or greater. Large for gestational age (LGA) generally implies a birthweight >90% for a given gestational age, and is dependent on both weight and gestational age with percentiles gener-ated from population-specific norms (see Table 18.1). By definition, the prevalence of LGA is fixed, but not all neonates at the upper extreme of size are pathologically large. Growth potential, growth rate, and gestational age at onset may be important considerations.
Macrosomia, like fetal growth restriction, has multiple potential causes, categorized into fetal or maternal factors (Box 18.2). Similar to fetal growth restriction, fetal factors include the genetic composition or inherent growth poten-tial of the individual, and genetic syndromes such as Beckwith-Wiedemann syndrome. Male fetuses are also more commonly affected than female fetuses.
Maternal factors include a history of macrosomia, maternal prepregnancy weight, weight gain during preg-nancy, multiparity, male fetus, gestational age greater than 40 weeks, ethnicity, maternal birth weight, maternal height, maternal age younger than 17 years, and a positive 50-g glu-cose screen with a negative result on the 3-hour glucose tol-erance test.
The magnitude of glucose intolerance during preg-nancy and specific measures of control are correlated with fetal weight and fetal fat mass. Lipids are also associated with fetal size, with triglycerides and free fatty acids posi-tively correlated to birth weight, and triglycerides independently associated with LGA infants.
Maternal body composition and body mass index are major determinants of insulin sensitivity, independent of hypertension and pregestational or gestational diabetes. Also, maternal weight gain and pregravid weight contribute to the variance in fetal birth weight. Finally, increased parity is associated with larger babies.
Macrosomia is associated with both increased maternal and fetal/neonatal risks. A patient with a macrosomic fetus has an increased risk of cesarean delivery, because of labor abnormalities. The risk of postpartum hemorrhage and vaginal lacerations are also elevated with macrosomia. Maternal infections associated with macrosomia include urinary tract infection in women undergoing elective cae-sarean section and puerperal fever in women undergoing a trial of labor. Risks to the fetus are shoulder dystocia and fracture of the clavicle, although brachial plexus nerve injury is rare. Macrosomic infants also have an increased risk for lower Apgar scores.
Other neonatal risks are partially dependent on the underlying etiology of macrosomia, such as maternal obesity or diabetes, and may include an increased risk of hypothermia, hyperbilirubinemia, hypoglycemia, prema-turity, and stillbirth. The relationship between gestational age and fetal size is important. Macrosomic preterm infants remain at risk for complications of prematurity. Size and extent of maturity are independent. Long-term risks include overweight or obesity in later life, again illus-trating that intrauterine growth may predict the founda-tion of many aspects of lifelong physiologic function.
Because the diagnosis of macrosomia is based on an esti-mated fetal weight above the 90th percentile and becomes increasingly imprecise at later gestational ages, careful dating of pregnancy is important. The two primary meth-ods for clinical estimation of fetal weight are Leopold maneuvers (abdominal palpation; see Fig. 9.7) and mea-surement of the height of the uterine fundus above the maternal symphysis pubis.
Measurement of the symphysis–fundal height alone is a poor predictor of fetal macrosomia and should be combined with clinical palpation of (Leopold maneuvers) to be useful.
Clinical findings may be combined with ultrasound to diagnose macrosomia. Ultrasound-derived estimates of fetal weight are obtained by entering the measurements of various fetal body parts, usually including the abdominal circumference, into one of several popular regression equa-tions. However, most of the regression formulas currently in use are associated with significant errors when the fetus is macrosomic. The superiority of ultrasound-derived esti-mates of fetal weight over clinical estimates has not been established.
The true value of ultrasound in management of macrosomia is its ability to rule out the diagnosis.
Differential diagnosis of an enlarged uterus includes a large fetus, more than one fetus (multiple gestation), extra amni-otic fluid (polyhydramnios), large placenta (molar preg-nancy), or large uterus (uterine leiomyomata, other gynecologic tumor, or uterine anomaly).
For mothers without diabetes, no clinical interventions designed to treat or curb fetal growth when macrosomia is suspected havebeen reported. Current evidence does not support earlydelivery for macrosomia alone, because induction of labor does not decrease maternal and neonatal morbidity; it does increase the rate of cesarean deliveries. In addition, the data do not support a specific estimated fetal weight at which women should undergo elective cesarean delivery.
Given the limitations of ultrasound estimations and the asso-ciation with increasing injury with increasing infant weight, the American College of Obstetricians and Gynecologists rec-ommends that a cesarean delivery should be offered for esti-mated fetal weights greater than 5000 g in women without diabetes and greater than 4500 g in women with diabetes.
Various techniques can be used to facilitate vaginal delivery in the case of shoulder dystocia, such as exaggerated flexion of the thighs (McRoberts maneuver), suprapubic pressure, various rotations, episiotomy, delivery of the posterior arm, and intentional clavicular fracture. The Zavanelli maneu-ver, cephalic replacement with subsequent cesarean deliv-ery, has yielded mixed results. A prolonged second stage of labor or arrest of descent in the second stage is an indica-tion for cesarean delivery. Postpartum or neonatal manage-ment depends on gestational age and underlying etiology.