An international study has found that abortions and miscarriages do not increase a woman's risk for breast cancer.
There is one relationship between pregnancy and breast cancer that is well documented, according to University of Oxford researcher Valerie Beral.
"For a long time it's been known that pregnancies that go through and end as a birth actually protect against breast cancer," she said.
In fact, additional births reduce the risk of breast cancer even more, although each successive one does not have as big an effect as the first birth.
But Dr. Beral says scientists had wondered if ending pregnancies early by abortion or miscarriage had the opposite effect of increasing the risk of breast cancer.
"There were a few individual studies that started to claim that induced abortions in particular might increase the risk of breast cancer," she said. " No one has ever said this about miscarriages, but there was this possibility. So this was something that was raised as a possibility, but had never really been investigated thoroughly in a systematic way."
To determine the truth, the British charity Cancer Research UK funded a worldwide project that did attack the question systematically. Researchers co-ordinated by Professor Beral and her colleagues reviewed 53 studies of 44,000 breast cancer patients that had been conducted in 16 nations with liberal abortion laws.
These included the English-speaking nations, Europe, Kenya, Japan, China, Thailand, Chile, and Mexico.
After combining and tabulating all the data, the investigators found no link between a terminated pregnancy and breast cancer.
"Overall we found that pregnancies that end both in miscarriages and induced abortions do not increase the risk of breast cancer," she said.
And, as they report in the journal The Lancet, the number of abortions and miscarriages a woman had did not matter.
Breast cancer surgeon Paul Tartter at St.Luke's-Roosevelt Hospital in New York says he is not surprised by the finding. He points out that women who have had abortions share something in common with women who have given birth. They both have had fewer menstrual cycles than women who have never been pregnant -- and fewer cycles means less breast cancer risk.
"So if you are to have an abortion, because you had to get pregnant before having the abortion, in fact you should be lowering your risk a little bit compared to not ever being pregnant at all. So from a biological standpoint, this is an outcome I would have predicted," he said.
Why had some earlier studies shown a connection between abortion, miscarriages, and cancer? The investigators who conducted the review study say that this may have been because women with breast cancer are more likely than healthy women in studies to disclose prior abortions, therefore distorting the results. Dr. Tartter explains why this is so.
"Basically when a patient is diagnosed with breast cancer and she's been interviewed by a whole bunch of physicians and health professionals and asked all the details about her health care over her entire life, she's much more likely to remember she had an abortion years ago than if you asked somebody who is otherwise healthy who is your control patient who is not being asked all those other questions," he said.
As a result, the review paper in The Lancet did not include studies in which women were asked their abortion histories after being diagnosed for breast cancer. For a fairer comparison to healthy women, it reviewed only those in which the cancer patients had their abortion histories taken before.