Yeah--there's a factor here that messes up the data. Namely, that pregnancy to term is associated with a *reduction* in breast cancer, the earlier the pregnancy the bigger the effect.
This is the reason behind the "pro-life" claim that abortion causes breast cancer--no, it simply doesn't provide the protection. Never-pregnant vs had-abortion is the same risk. Hormonal contraception is more effective and will reduce the oops rate and thus would be expected to show a positive correlation with breast cancer.
(And the oops rate is also why for a young non-smoker the risk of the pill is *negative* even if the response to an oops is an immediate abortion. Carrying it to term increases the effect by at least an order of magnitude.)
> These calculations were then adjusted to take into account established risk factors such as body mass index (BMI), number of recorded births, and the time since a woman’s last birth.
This is the reason behind the "pro-life" claim that abortion causes breast cancer--no, it simply doesn't provide the protection. Never-pregnant vs had-abortion is the same risk. Hormonal contraception is more effective and will reduce the oops rate and thus would be expected to show a positive correlation with breast cancer.
(And the oops rate is also why for a young non-smoker the risk of the pill is *negative* even if the response to an oops is an immediate abortion. Carrying it to term increases the effect by at least an order of magnitude.)