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A high level of endogenous estrogen in utero has been hypothesized to be a possible risk factor for breast cancer. We used information from two population-based case-control studies to investigate the relation between perinatal factors and risk of invasive breast cancer among women age 21-45 years (746 cases, 960 controls) and women age 50-64 years (401 cases, 439 controls). Breast cancer cases were ascertained through a population-based cancer registry, and controls were selected by random digit dialing. After adjustment for age, menopausal status, and maternal smoking, the birthweight-breast cancer association in women age 21-45 years followed a J-shaped curve, with women whose birthweight was less than 2,500 gm [odds ratio (OR) = 1.3; 95% confidence interval (CI) = 0.9-2.0] and 4,000 gm or more (OR = 1.7; 95% CI = 1.1-2.5) at increased risk. Women age 50-64 years who were 4,000 gm or more at birth appeared to be at slightly reduced risk of breast cancer (OR = 0.6; 95% CI = 0.3-1.1). With the exception of maternal smoking, there was little effect of other perinatal factors on breast cancer risk in either group. These results support the hypothesized association between intrauterine estrogen exposure and subsequent risk of breast cancer.

Objective To ascertain whether maternal twinning influences the previously described association between the birthweight of singleton mothers and their infants. Design and subjects The association between the birthweight of singleton parents and their offspring and that between twin parents and their offspring was compared using data from the 23‐year‐old sample of the 1958 British national birth cohort. The numbers available for full comparison were 1027 female and 611 male singleton cohort members, and 26 female and 17 male twin cohort members and their first singleton livebirths. Results Of the mothers who had been twins, half had been of low birthweight, and overall their mean weight was 700 g less than that of their singleton counterparts. Nevertheless, the mean birthweight of babies of twin mothers was 133 g, and of twin fathers 94 g, greater than of babies of corresponding singletons. For female, but not male, parents this difference persisted after adjustment for confounding variables. Conclusions We suggest two possible reasons for the difference between the weight of babies of twin and singleton mothers. Firstly, the growth of twins becomes retarded late in pregnancy, possibly after a period critical in determining long‐term reproductive effects. Secondly, the twin survivors were of higher birthweight than the original twin cohort, whilst the singleton survivors were more representative of all singleton births, thus introducing a possible bias.

The early-life childhood and adolescent environment is associated with the risk of AD.

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