Table 3 Birth weight among infants born to Swedish hairdressers and their sisters
RestrictionsSubgroupHairdressersSistersp Value
NoMean birth weight (g)95% CINoMean birth weight (g)95% CI
NoneAll*622334633439 to 3488838834663443 to 34890.821
Same parents*622334553430 to 3480491434603432 to 34880.650
Hairdressers*434734663431 to 3500485734673434 to 35000.913
Born in 1982 to or laterAll*554934733445 to 3501719034733447 to 35000.996
Same parents*554934603432 to 3488444934623432 to 34930.858
Hairdressers*420634643428 to 3500424434643429 to 35000.992
Born in 1982 to or laterAll†464734063375 to 3437577634293400 to 34580.070
Same parents†464734073373 to 3440363934303394 to 34670.082
Hairdressers†357233873348 to 3427341134193380 to 34580.033

  • Mean birth weights with 95% confidence intervals (CI), as well as p values, were calculated using a mixed models approach, taking parity and siblingship into account. Results are presented for the entire cohorts, and from subgroup analyses including only sibling pairs where it was known for certain that they had both parents in common, as well as for sibling pairs where the woman from the hairdresser cohort had worked as a hairdresser during at least one pregnancy. Information on all outcomes was collected from the Medical Birth Registry (MBR).

  • *Adjusted for gender and maternal age.

  • †Adjusted for gender, maternal age, maternal height, and smoking.