Abstract
BackgroundA longer interval between age at menarche and 1st pregnancy is a known breast cancer risk factor. Whether physical activity during this period can mitigate cancer risk is unknown.
 Methods. We analyzed 65569 parous women in the Nurses' Health Study II who reported their physical activity at ages 12 to 34 in 1997. Multivariable relative risks (RRs) were estimated using Cox proportional hazards regression.
 Results. 2078 breast cancer cases occurred between 1989 and 2011. As expected, a longer interval between menarche and 1st pregnancy was associated with greater breast cancer risk (RR=1.04 per additional yr, 95% CI 1.03‐1.05); overall, this relationship did not vary by physical activity level before 1st pregnancy. However, an inverse association between physical activity before 1st pregnancy and breast cancer risk was observed for women with a menarche to 1st pregnancy interval of <10 yrs (RR=0.59 for highest vs lowest quartile of activity, 95% CI 0.39‐0.90), with no association in women with longer intervals.ConclusionPhysical activity before 1st pregnancy did not modify the association between the interval from menarche to 1st pregnancy and breast cancer risk, except among women with an interval of <10 yrs, in which early physical activity was inversely associated with breast cancer risk.Supported by: NIH U54 CA155626, CA155850, CA155496 and U01 CA116850
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