I feel for those of you who's babies have suffered shoulder dystocia. It is quite rare and difficult to predict, with an incidence ranging from 0.15% to 2.1% of all births.
Here' some information from:
Current Opinion in Obstetrics and Gynecology
(C) 2006 Lippincott Williams & Wilkins, Inc.
Volume 18(2), April 2006, pp 123-128.
Shoulder dystocia and injuries: prevention and management [Maternal-fetal
medicine]
"Shoulder dystocia has been aptly described as being infrequent, unanticipated and unpredictable [1], features that contribute to the nightmare awaiting the person attending the delivery. Timely intervention
allows rapid delivery of the baby in the majority of cases; however, the
condition is associated with a significant risk of fetal hypoxia and injury
to both mother and baby."
Prediction and prevention of shoulder dystocia:
"The assessment of women for the risk of shoulder dystocia is a continuous
process, beginning even before pregnancy, when factors such as maternal
diabetes mellitus, obesity or a history of shoulder dystocia or macrosomia
in a previous pregnancy warn us of a significantly increased risk of
shoulder dystocia. Antenatal risk factors consistently reported are fetal
macrosomia, diabetes mellitus in pregnancy, excessive maternal weight gain
and post-date pregnancy. It is likely that absolute birthweight threshold is
not as important as birthweight percentile for specific populations. In an
Asian population, a birthweight above 3600 g (almost the 90th percentile for
the population) was associated with a relative risk 16.1 times higher for
the occurrence of shoulder dystocia compared to pregnancies resulting in the
delivery of an infant weighing less than 3600 g [18]."
"Intrapartum events such as a prolonged labour and the need for instrumental
delivery should alert us to the risk of shoulder dystocia [6]. In two recent
retrospective analyses, the risk of shoulder dystocia among infants delivered by vacuum extraction was significantly higher than those delivered by forceps."
"Despite the awareness of these well established and thoroughly reviewed risk
factors, our ability to predict the occurrence of shoulder dystocia in any
one particular pregnancy is deficient. combination."
Please also consider the risk to mother and baby from Caesarean section:
www.ican-online.org/pregnancy/cesarean-fact-card
Another study concluded:
"Historic obstetric risk factors for shoulder dystocia are not useful predictors for the event. Furthermore, although shoulder dystocia was observed more frequently with increasing birth weight, current limitations in estimating birth weight antenatally with accuracy preclude its practical use as a reliable predictor."
This means we are not very accurate at predicting babies weight by scans etc.