- New England Journal of Medicine (1994)
- American Journal of Epidemiology (2000)
- Cancer Causes Control (2002)
Why haven't you heard about this? First, because it's only temporary, and second, because the overall effect of childbirth is to reduce a women's lifetime risk. Young women have a very low risk of breast cancer, so a small increase in a very low risk is not a big problem. Older women have a much greater risk of breast cancer, so a long-term reduction in risk makes a big difference. That's why having children is generally associated with lower breast cancer risk.
That's good news. But it leaves us with two questions. Why does breast cancer risk go up after childbirth? Why does it go down again after 15 years?
Jose and Irma Russo, of Fox Chase Cancer Center in Philadelphia, have done countless studies on the physiology of breast tissue. They have demonstrated that breast tissue specializes during the latter stages of pregnancy, leaving mature tissue at a lower risk of cancer. The early stages of pregnancy, by contrast, are a time of rapid cell division. (Women who have had babies know about this--tender breasts are one of the earliest signs of pregnancy.) These rapidly-replicating cells are a higher risk of mutation.
The prime suspect for the transient risk after childbirth is estrogen. Estrogen helps prepare a woman's body for birth. It is the "gasoline" that keeps the human race going--but gasoline can be dangerous. All it takes is a spark to create an explosion. If you add enough estrogen to just one abnormal breast cell you can grow a tumor.
The good news is that the hormones of later pregnancy may actually stop some tumors in their tracks. The research to date is consistent with research on rats which indicates that tumors which have already begun to grow are redirected into normal tissue. (If you enjoyed biology in high school, read this article--it spells out the mechanism by which stem cells of one type are converted to another, and zeroes in on the precise tissue types that are at risk in early pregnancy.) If estrogen is the "gasoline" that catches fire, the hormones of later pregnancy are "rain" that puts fires out.
These are plausible answers to our two questions (why does risk go up after childbirth? Why does it go down after 15 years?), but if they turn out to be correct, they lead directly to a third question. What happens if a woman's breast cancer risk is raised by early pregnancy without the protective effect of later pregnancy? What if there's gasoline but no rain?
Research on rats proves that an interrupted pregnancy confers no protective effect. How about humans? If women have an elevated risk after any pregnancy (not just a full-term pregnancy), then women who choose abortions should have an elevated risk for at least 15 years after the procedure, just like other women who were pregnant. Carefully designed studies of breast cancer in young women find exactly that. Dr. Janet Daling, of Seattle's Fred Hutchinson Cancer Research Center, found:
Among women who had been pregnant at least once, the risk of breast cancer in those who had experienced an induced abortion was 50% higher than among other women (95% CI = 1.2-1.9). While this increased risk did not vary by the number of induced abortions or by the history of a completed pregnancy, it did vary according to the age at which the abortion occurred and the duration of that pregnancy. Highest risks were observed when the abortion was done at ages younger than 18 years—particularly if it took place after 8 weeks' gestation—or at 30 years of age or older.To answer the question posed by the title: childbirth does not cause cancer. Pregnancy does--in the sense that it adds "gasoline" that can burst into flame. Full-term pregnancy is the rain that puts fires out and reduces the risk of fire thereafter.
