Although a history of schizophrenia in a parent or sibling is associated with the highest relative risk of having the disease, the place and season of birth account for many more cases on a population basis.

Delivery by c-section is associated with a moderate risk increase for allergic rhinitis, asthma, hospitalization for asthma, and perhaps food allergy/food atopy, but not with inhalant atopy or atopic dermatitis. The increased use of c-section during the last decades is unlikely to have contributed much to the allergy epidemic observed during the same period.

Conclusion:A diagnosis of breast cancer less than 2 years after having given birth is associated with a particularly poor survival irrespective of the stage of disease at debut. Therefore, a recent pregnancy should be regarded as a negative prognostic factor and should be considered in counselling these patients and in the decisions regarding adjuvant treatment.

Our results are compatible with the hypothesis that a high birth weight is associated with an increased risk of ALL.

The association between birth weight and childhood leukemia suggests the importance of intrauterine factors. A plausible explanation may be that increasing birth weight is associated with a higher rate of cell proliferation and/or a larger number of precursor cells being at risk of malignant transformation. The inverse association between birth order and ALL risk among 0-4 year olds was weak, but it was compatible with the hypothesis that delayed exposure to infection may increase the risk of ALL in this age group. The association of childhood AML with birth order and sibship size at young ages deserves further attention in the search for environmental factors that affect childhood AML risk.