Several large US population-based cohort studies have shown a significant relationship between progestational BMI and macrosomia, which is defined as a birth weight
> 4,000 g or above the 90th percentile [40, 44-46]. Risk for having a macrosomic infant appears to increase in mothers with degree of excess weight. For example, in the multicenter study by Weiss et al. , the incidence of macrosomia was 8.3% in nonobese women, 13.3% in obese women, and 14.6% in morbidly obese women. In the study by Baeten et al. , the odds of having a macrosomic infant were 1.2 in women who were normal weight, 1.5 in women who were overweight, and 2.1 in women who were obese. The analysis excluded women with chronic hypertension, pregestational and gestational diabetes, and preeclampsia. National and international trends in North America  and Europe [48, 49] report an increase in incidence of large for gestational age infants, and implicate rising trends in maternal obesity and diabetes, and declining trends in maternal smoking as causal factors.
Macrosomia is a well-established risk factor for shoulder dystocia and birth trauma. The risk is directly related to birth weight and increases substantially with birth weight
> 4,500 g. Injury to the brachial plexus is reportedly rare but increases substantially (tenfold) with birth weights > 4,500 g . Bassaw et al.  conducted a 9-year review of over 100 cases of shoulder dystocia from among ~47,000 vaginal deliveries. As compared with infants weighing between 3,500 and 3,999 g, these researchers found a 2.2% higher frequency of shoulder dystocia in infants weighing 4,000-4,499 g at birth. The frequency of shoulder dystocia increased by 7.1% with birth weights > 4,500 g. Obesity was the most important identifiable predisposing factor for shoulder dystocia and occurred in 35.9%.
Risk for late-gestation fetal demise is also greater among obese women compared with their normal weight counterparts. Population-based studies in England , Sweden , Norway , and Canada  showed a significant relationship between obesity and risk of late fetal death (stillbirth occurring after 28 weeks of gestation) even after adjusting for gestational diabetes, gestational hypertension, preeclampsia, maternal age, and parity. Given the dire consequences of progestational obesity, intervention strategies for helping women achieve a healthy prepregnant BMI are urgently needed.
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