A recent unpublished but widely reported study found that women who gained 38 pounds or more during pregnancy had a 40% greater likelihood of developing breast cancer decades later. When I read about the study, I found myself mentally tabulating my weight gain during pregnancy and feeling dread about something now way beyond my control.
Some people I spoke to were confused and concerned because they thought that having been pregnant lowered their risk of breast cancer.
But the study, presented earlier this month at a meeting of the American Assn. for Cancer Research in San Francisco, is really an example of how an interesting study should be seen as a spur to further research, not as a definitive finding.
In the study, Finnish researchers looked at the records of two sets of women. First they looked at the weight gain during pregnancy of 17,360 women who had been pregnant between 1990 and 1993 and found 98 who developed breast cancer before they reached menopause.
Then they looked at weight gain in 4,090 women who had been pregnant between 1954 and 1963. Of these, 185 went on to develop breast cancer many years later, when they were post-menopausal. They found that while there appeared to be no relationship between weight gain and developing pre-menopausal breast cancer, that was not true for post-menopausal cancer. Those older women in the second group who had gained 38 or more pounds during pregnancy had a 40% increased risk of developing breast cancer years later compared with those who gained between 26 and 31 pounds, said the lead researcher, Leena Hilakivi-Clarke, an associate professor of oncology at Georgetown University in Washington, D.C.
There are essentially two theories about why this may be the case, neither of which can be "proved" in this study. Not just the ovaries produce estrogen; so do fat cells. One theory is that greater weight gain during pregnancy exposes already estrogen-drenched and rapidly changing breast tissue to even more estrogen. This could alter breast tissue during a time when it is particularly susceptible.
Dr. Marc Citron, a member of the steering committee of the Long Island Breast Cancer Study Project, a federal study looking at the possible relationship between pesticides and breast cancer, said the onset of menstruation, pregnancy and menopause are three times in a woman's life when breast tissue undergoes rapid change. "There may be certain windows ... when women may be more susceptible to a carcinogenic stimulus," said Citron, chief of oncology at ProHealth, a medical group in Lake Success, N.Y. This may be a relatively minor factor "within the whole totality of different exposures" that doesn't come into play until a woman is older, he said.
Another possibility is that it is being overweight per se, not the pregnancy, that confers the risk. Experts have long observed that overweight or obese women have a higher risk of developing breast cancer after menopause. The theory is that fat releases estrogen--even after the ovaries shut down--and more years of estrogen exposure mean more chances for a genetic mistake that could lead to cancer. In fact, one of the few known ways a woman can reduce her risk of post-menopausal breast cancer is to keep a healthy weight, experts say. The problem is that in this study it is impossible to deduce which theory might be correct because most of the women who developed breast cancer were still overweight years after their pregnancy, Hilakivi-Clarke said.
"We don't know if women who gained at the highest rate and then lost it all ... still have an increased risk," she said. "I can't answer that." Others interviewed said that's a key question.
"We know that [too much] weight is a risk for post-menopausal breast cancer," said Dr. Harold Burstein, a breast oncologist at Dana-Farber Cancer Center in Boston. While Burstein said too much weight gained during pregnancy might prove to have a negative impact on breast cells, the study should be seen as "suggestive and interesting" only. Dr. Jeanne Petrek, a breast surgeon at Memorial Sloan-Kettering Cancer Center in Manhattan, agreed. "It could be that what we're looking at is these women who gained a lot of weight at pregnancy and stayed heavy," she said, adding that this is when many women gain and do not lose excess weight.
How does this balance out with the belief that being pregnant lowers the risk of getting breast cancer? It appears that age is key. According to one major study, women who had a first full-term pregnancy before age 20 were half as likely to develop breast cancer as women who never had a baby or those who had their first child at age 35 or older. The bottom line, Petrek and others said, is that women should strive to maintain a reasonable weight during pregnancy and then take off excess pounds afterward.
Ridgely Ochs writes for Newsday, a Tribune company.