Here is an article on Shoulder Dystocia-hope this helps.
Shoulder dystocia occurs in approximately one half of one percent of all deliveries. Given that there are 4 million babies born each year in the United States, this delivery complication will be experienced by roughly 20,000 women a year. The larger the baby, the more likely it is to occur. However, even with very large babies shoulder dystocia occurs only occasionally and sporadically. Therefore a physician never knows when it will be encountered.