The incidence of shoulder dystocia is generally reported to be between 0.5 % and 1.5% with scattered reports listing values both higher and lower. Those studies involving the largest number of deliveries have usually found the rate of shoulder dystocia in a general population to be 0.5% - 0.6%. The "true" incidence of shoulder dystocia, however, is very much dependent upon how it is defined, how it is reported, and the characteristics of the population being measured.

The accuracy of reporting is an important variable in shoulder dystocia statistics. Many obstetricians are reluctant to write down in their delivery notes that a shoulder dystocia has occurred for fear that this will be a red flag attracting a malpractice suit should it later turn out that the baby has suffered an injury. Some studies have shown that only 25% to 50% of shoulder dystocias -- as noted by objective observers in a delivery room -- are recorded by the delivering physician.

How one defines shoulder dystocia, of course, affects its reported incidence. Some obstetricians will only report a delivery as involving shoulder dystocia if they had to employ specific maneuvers to deliver the baby's anterior shoulder. Others will record shoulder dystocia if there is any delay in the emergence of the shoulder following delivery of the head. In some cases a physician will only record "shoulder dystocia" when a fetal injury has occurred.

Finally, the characteristics of the delivery group being measured will affect statistics on shoulder dystocia. A study evaluating the incidence of shoulder dystocia utilizing only large babies or only infants of diabetic mothers as subjects will have a much higher reported incidence of shoulder dystocia than if the population were a general one containing both small and large babies and the normal percentage of mothers having diabetes.

Several of the more recent studies have shown a slightly higher incidence of shoulder dystocia than has been recorded in the past, reaching just above 1% of all deliveries. The question has therefore been asked, "Is the rate of shoulder dystocia increasing?" While there is as yet no definitive answer to this question, several hypotheses have been given to explain this possible trend:

1. On average babies are significantly larger then in years past. The percentage of very large baby's (>4000gms) in one study has gone up 300% between 1970 and 1988.

2. Over the last several decades there has been a marked increase in average maternal weight, average maternal weight gain during pregnancy, and the number of diabetic women having babies. All of these factors could be expected to increase the incidence of shoulder dystocia.

3. The increased focus of attention among obstetricians about shoulder dystocia deliveries may have heightened awareness about it and increased reporting of it.

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