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Objective The study was originally designed to identify the risk factors that could predict those difficult instrumental deliveries resulting in birth trauma and birth asphyxia.

Design A prospective study on all singleton deliveries in cephalic presentation with an attempt of instrumental delivery over a 12-month period (13 March 2000 to 12 March 2001).

Setting A local teaching hospital.

Sample Six hundred and seventy deliveries.

Methods A codesheet was designed to record the demographic data, characteristics of first and second stages of labour and neonatal outcome. In particular, the doctor had to enter the pelvic examination findings before the attempt of instrumental delivery.

Main outcome measures Birth trauma and birth asphyxia.

Results There was a significant reduction in the incidence of birth trauma and birth asphyxia related to instrumental deliveries during the study period (0.6%) when compared with that (2.8%) in the pre-study period (1998 and 1999) (RR 0.27, 95% CI 0.11–0.70). There was more trial of instrumental deliveries in the operating theatre although this was not statistically significant (RR 1.19, 95% CI 0.88–1.60). The instrumental delivery rate decreased during the study period (RR 0.88, 95% CI 0.82–0.94). The caesarean section rate for no progress of labour, the incidence of direct second stage caesarean section and the incidence of failed instrumental delivery did not increase during the study period.

Conclusions Apart from the merits of regular audit exercise and increasing experience of the staff, the Hawthorne effect might be the major contributing factor in the reduction of birth trauma and birth asphyxia related to instrumental deliveries during the study period.