Williams Manual of Pregnancy Complications, 23 ed.

CHAPTER 39. Macrosomia

Macrosomia is a term used rather imprecisely to describe a very large fetus/neonate. There is general agreement among obstetricians that newborns weighing less than 4000 g are not excessively large but a similar consensus has not been reached that permits a precise definition of macrosomia.


Several definitions of macrosomia are in general clinical use. Two commonly used definitions are based upon the mathematical distribution of birth weight. Birth weights exceeding the 90th percentile for a given gestational week are used as one threshold for macrosomia. On the other hand, birth weights two standard deviations above the mean (97th percentile) are also used to define excessive fetal growth. For example, the birth weight threshold at 39 weeks would be approximately 4500 g (97th percentile) rather than 4000 g (90th percentile). Absolute birth weight exceeding a specific threshold is also commonly used to define macrosomia. For example, weight exceeding 4000 g (8 lb 13½ oz) is a frequently used threshold. Others include 4250 g or even 4500 g (almost 10 lb). As shown in Table 39-1, birth weights of 4500 g or more are rare (1.5 percent). The American College of Obstetricians and Gynecologists (Practice Bulletin No. 22, 2000) has concluded that the term macrosomia is an appropriate designation for fetuses who, at birth, weigh 4500 g or more.

TABLE 39-1. Birth Weight Distribution of 171,755 Liveborn Infants at Parkland Hospital Between 1998 and 2008



Known maternal risk factors are identified in only 40 percent of women who deliver macrosomic infants. As shown in Table 39-1, the incidence of maternal diabetes increases as birth weight above 4000 g increases (see Chapters 71 and 72). However, it should be emphasized that maternal diabetes is associated with only a small percentage of such large infants. Among macrosomic fetuses of diabetic women, there is a greater shoulder circumference—a consequence of which is a greater risk of shoulder dystocia at vaginal birth. Listed in Table 39-2 are several other factors that favor the likelihood of a large fetus. These factors are additive.

TABLE 39-2. Some Factors Associated with Fetal Macrosomia



Currently, an accurate estimate of excessive fetal size is not possible; consequently, the diagnosis of macrosomia can be confirmed only after delivery. Maternal obesity further adds to the inaccuracy in clinical estimates of fetal weight by physical examination.

Numerous attempts have been made to improve the accuracy of fetal weight estimations by analysis of various measurements obtained by ultrasonography. A number of formulas have been proposed to estimate fetal weight using ultrasonic measurements of head, femur, and abdomen (see Chapter 9). The estimates provided by these computations, although reasonably accurate for predicting the weight of small, preterm fetuses are less valid in predicting the weight of very large fetuses. A formula that gives estimates of fetal macrosomia with sufficiently accurate predictive value has not been derived. For example, most ultrasonic estimates of fetal weight are about ±15 percent of the actual birth weight.


Precise knowledge of fetal weight might permit the avoidance of vaginal delivery in women whose labor would most likely be arrested because of true fetopelvic disproportion or delivery complicated by shoulder dystocia. There are several controversial approaches to preventing these delivery complications of macrosomia.

“Prophylactic” Labor Induction

Some have proposed induction of labor upon diagnosis of macrosomia in nondiabetic women as a way to avoid further fetal growth and thereby reduce potential delivery complications. However, labor induction has not been shown to decrease the rate of cesarean delivery or shoulder dystocia by preempting further fetal growth.

Elective Cesarean Delivery

A policy of elective cesarean delivery for ultrasonically diagnosed fetal macro-somia compared with standard obstetrical management has been reported to be medically and economically unsound. However, a policy of elective cesarean delivery in diabetic women with macrosomic fetuses may be a tenable approach. A protocol of routine cesarean delivery for ultrasonic fetal estimates of 4250 g or greater in diabetic women has been reported to significantly reduce the rate of shoulder dystocia.

Prevention of Shoulder Dystocia

A major concern for delivery of macrosomic infants is shoulder dystocia and attendant risks of permanent brachial plexus palsy. Shoulder dystocia occurs when the maternal pelvis is of sufficient size to permit delivery of the fetal head, but not large enough to allow delivery of the very-large-diameter fetal shoulders (see Chapter 16). In this circumstance, the anterior shoulder becomes impacted against the maternal symphysis pubis. Even with expert obstetrical assistance at delivery, stretching and injury of the brachial plexus of the affected shoulder may be inevitable. Fortunately, fewer than 10 percent of all shoulder dystocia cases result in a permanent brachial plexus injury.

In light of the fact that most cases of shoulder dystocia cannot be predicted or prevented, a policy of planned cesarean delivery on the basis of suspected macrosomia in the general population is unreasonable because of the number and cost of additional cesarean deliveries. Planned cesarean delivery may be a reasonable strategy for diabetic women with estimated fetal weights exceeding 4250 to 4500 g.

For further reading in Williams Obstetrics, 23rd ed.,

see Chapter 38, “Fetal Growth Disorders.”