- Women with borderline glucose tolerance, as well as those with normal glucose tolerance but mildly hyperinsulinemic fetuses, are at risk of delivering macrosomic infants.
- Maternal risk factors include obesity, excessive weight gain, and a history of delivering a macrosomic infant.
- The precision of ultrasound measurements declines as fetal weight increases.
- Estimating birth weight does not accurately predict the risk of brachial plexus injury.
- Neither routine cesarean delivery nor induction of labor is appropriate routine management for suspected macrosomia.
Every clinician would like to avoid vaginal delivery of a macrosomic infant and the attendant potential for shoulder dystocia and permanent brachial plexus injury. Unfortunately, research findings offer little specific guidance, and we continue to wrestle with these dilemmas:
- We cannot accurately identify which fetuses are macrosomic.
- We cannot accurately predict serious morbidity in these fetuses.
- Evidence does not support a policy of avoiding vaginal delivery for all macrosomic fetuses.
Patient care is thus based on estimating the likelihood of macrosomia and its complications, evaluating the risks and benefits of cesarean versus vaginal delivery in each woman, and being prepared for optimal labor management.
Dilemma 1How to identify a macrosomic fetus?
Ultrasound measurements are reasonably accurate for estimating the weights of smaller fetuses, but precision drops off as fetal weight increases. Studies using abdominal palpation and fundal height to estimate the risk of macrosomia report sensitivities of 10% to 43% and positive predictive values of 28% to 53%.1,2
One investigation with results typical of other studies found that when birth weight exceeded 4,500 g, only 50% of fetuses weighed within 10% of the ultrasound estimate.3 Both clinical and ultrasound estimates either overestimate or underestimate birth weight to such a degree as to limit their clinical utility.
Maternal factors increase the risk of macrosomia (TABLE 1).
- Obese women are more likely to have large infants than women with normal body mass.4
- Excessive weight gain during pregnancy has been shown to increase risk of accelerated fetal growth.
- History of macrosomia is another leading risk factor for a large birth-weight infant.
- Maternal glucose intolerance is an established risk factor: Fetal growth is accelerated in women with poorly controlled type 1 or 2 diabetes mellitus. Less widely known is the fact that women with borderline glucose tolerance, as well as those with normal glucose tolerance but mildly hyperinsulinemic fetuses, have an increased risk of delivering macrosomic infants.
A study comparing pregnant women with and without insulin-dependent diabetes found that neonatal macrosomia was best correlated with umbilical total insulin, free insulin, and C-peptide levels.5
Fetal hyperinsulinemia. Many other studies corroborate the notion that fetal hyperinsulinemia is a major influence on excessive fetal growth. For example, Hoegsberg et al6 found that cord-blood plasma insulin levels in macrosomic newborns were twice those of normosomic infants (all neonates were of nondiabetic mothers). Another study comparing 207 macrosomic infants with 200 controls demonstrated that the macrosomic infants had higher levels of plasma insulin and insulin-like growth factor-1.7
Risk factors for fetal macrosomia
|Excessive weight gain during pregnancy|
|History of macrosomia|
|Maternal glucose intolerance or borderline tolerance|
Dilemma 2How likely is serious morbidity?
Fetal macrosomia poses a threat to mother and newborn alike. Once fetal birth weight exceeds the 90th percentile, maternal morbidity increases linearly. Not surprisingly, labor abnormalities are more common and, when birth weights exceed 4,500 g, cesarean delivery rates for laboring women double.8 Vaginal and perineal lacerations and postpartum hemorrhage are also more common following vaginal delivery of a large fetus compared with a newborn of normal size.
Predicting risk of shoulder dystocia. What worries the obstetrician most, however, is the potential for shoulder dystocia and permanent brachial plexus injury. Nesbitt et al9 examined nearly 176,000 vaginal births of infants weighing more than 3,500 g, all occurring in 1 year in California, and found risk factors for shoulder dystocia included maternal diabetes, increased birth weight, and assisted delivery (FIGURE). Some specifics:
- In unassisted births not complicated by diabetes, the rate of shoulder dystocia was 5.2% for infants weighing 4,000 to 4,250 g; this rate rose to 9.1% for newborns weighing 4,250 to 4,500 g and jumped to 21.1% for those weighing 4,750 to 5,000 g.
- In diabetic mothers, the risk of shoulder dystocia in unassisted births was 8.4% at birth weights from 4,000 to 4,250 g, rising to 23.5% at 4,750 to 5,000 g.
- When delivery included use of forceps or vacuum, the incidence of shoulder dystocia rose by about 35% to 45% in nondiabetic mothers.
Of course, it must be noted that numerous cases of shoulder dystocia develop in fetuses weighing less than 4,000 g (TABLE 2).