Correlation does not imply or prove causation.
When there is a reasonable explanation for the correlation (a disruption in hormones), it's reasonable to assume a causal relationship.
When a woman becomes pregnant, her breasts enlarge. This occurs because a hormone called estradiol, a type of estrogen, causes both the normal and pre-cancerous cells in the breast to multiply terrifically. This process is called “proliferation.” By 7 to 8 weeks gestation, the estradiol level has increased by 500% over what it was at the time of conception.If the pregnancy is carried to term, a second process called “differentiation” takes place. Differentiation is the shaping of cells into milk producing tissue. It shuts off the cell multiplication process. This takes place at approximately 32 weeks gestation.
If the pregnancy is aborted, the woman is left with more undifferentiated -- and therefore cancer-vulnerable cells -- than she had before she was pregnant. On the other hand, a full term pregnancy leaves a woman with more milk producing differentiated cells, which means that she has fewer cancer-vulnerable cells in her breasts than she did before the pregnancy.
In contrast, research has shown that most miscarriages do not raise breast cancer risk. This is due to a lack of estrogen overexposure. Miscarriages are frequently precipitated by a decline in the production of progesterone which is needed to maintain a pregnancy. Estrogen is made from progesterone, so the levels of each hormone rise and fall together during pregnancy.