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 emer gency caesarean section at term; 2. to determine whether the relation betwe en birthweight and caesarean section differed between male and female babie s; and 3. to determine what proportion of the increased rates of caesarean section could be related to greater birthweights. Design Retrospective population study. Data collected from Scottish Morbidi ty Record 1980 to 1996. Population Data All first singleton deliveries by emergency caesarean secti on 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 c aesarean section, with the lowest risk associated with weights in the range 3000-3500 g. Overall, males 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 < 4000 g were at increased risk of cae sarean section compared with similarly sized females. Between 1980 and 1996 , there were linear increases in the rate of caesarean section (from 7.1% t o 10.7%. r(2) = 0.8, P < 0.001) and median birthweight (from 3360 g in 1980 to 3420 g in 1996, r(2) = 0.8, P < 0.001). The population attributable fra ction of caesarean sections related to year of delivery 1981-1996 was not s ignificantly altered by adjusting for birthweight (22.3% vs 21.6%). Conclusions There is no evidence to suggest that increasing birthweights ha ve 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.