In conclusion, although the results regarding birth order and breast-feeding do not fit with Greaves' hypothesis, the study supports the hypothesis that early common infections may play a protective role in the aetiology of childhood leukaemia, although this effect was not more marked for common ALL.
For younger women, adjustment for maternal age, adult BMI, and household income revealed a weak, statistically nonsignificant, inverse relationship between higher birth order and odds of breast cancer, which did not differ appreciably by race.
These data are not consistent with the epidemiological evidence that birth weight increases with birth order, and the correlation of birth weight and breast cancer risk (Trichopoulos, 1990; Michels et al, 1996; Sanderson et al, 1996; Juntunen et al, 1997; Andersson et al, 2000; Kaijser et al, 2000).
The interviews included questions on early common infections, day-care attendance, breast-feeding, birth order and infantile diseases.
The only significant associations were an increasing risk for breast cancer by birth order and a decreasing risk for melanoma by birth order and, particularly, by family size.
Pregnancy-associated changes in BMI, and increased PCB clearance through breast-feeding, could result in associations between birth order and blood PCB concentrations.
Although the current meta-analysis found evidence that higher birth weights are associated with increased breast cancer risk, older maternal age and twin status were less convincingly related, and birth order and prematurity appeared unrelated.
Offspring covariates were birth weight, whether the newborn was preterm or small for gestational age (SGA; birth weight for gestation < 10th percentile) [ 20], birth order and gender, and the age when the child was introduced to cereals and any other kind of milk than breast milk.
Birth order was classified into two groups - first birth order and second birth order.
