A population study of birthweight and the risk of caesarean section: Scotland 1980-1996.

OBJECTIVES

1. To describe the relation between birthweight and risk of emergency caesarean section at term; 2. to determine whether the relation between birthweight and caesarean section differed between male and female babies; and 3. to determine what proportion of the increased rates of caesarean section could be related to greater birthweights.

DESIGN

Retrospective population study.

SETTING

Data collected from Scottish Morbidity Record 1980 to 1996.

POPULATION: All first singleton deliveries by emergency caesarean section and non-elective vaginal birth of live babies at 40 weeks of gestation (n = 120,854).

MAIN OUTCOME MEASURE: Delivery by emergency caesarean section.

RESULTS

There was a U-shaped relation between birthweight and the risk of caesarean section, with the lowest risk associated with weights in the range 3,000-3,500 g. Overall, male, were more likely to be delivered by caesarean section (relative risk = 1.2, 95% CI 1.2-1.3). The association between male sex and increased risk of caesarean section persisted after adjusting for birthweight, but only males weighing < 4,000 g were at increased risk of caesarean section compared with similarly sized females. Between 1980 and 1996, there were linear increases in the rate of caesarean section (from 7.1% to 10.7%, r2 = 0.8, P < 0.001) and median birthweight (from 3,360 g in 1980 to 3,420 g in 1996, r2 = 0.8, P < 0.001). The population attributable fraction of caesarean sections related to year of delivery 1981-1996 was not significantly altered by adjusting for birthweight (22.3% vs 21.6%).

CONCLUSIONS

There is no evidence to suggest that increasing birthweights have contributed to increasing rates of caesarean section in Scotland between 1980 and 1996 among singleton first births non-electively delivered at 40 weeks of gestation.