Women who survive breast cancer and become pregnant afterwards don't appear to be at any higher risk of dying from cancer, a new study says.
Doctors have long worried that because estrogen levels soar during pregnancy and many breast cancers are fuelled by estrogen, the hormonal changes could spur the disease's return in those who have already battled the disease.
For that reason, many breast cancer patients are counselled against getting pregnant after they recover.
But in research presented Friday at the European Breast Cancer Conference EBCC7 in Barcelona, experts said they looked at the numbers and found that pregnancy in breast cancer survivors does not seem to be linked with the disease's recurrence.
Dr. Hatem Azim of the Institute Jules Bordet in Belgium led a team that analyzed results from 14 studies, dating back almost 30 years, that followed more than 1,400 pregnant women with a history of breast cancer.
Azim's team compared those women to more than 18,000 women who had had breast cancer but did not later become pregnant.
They found that the women who got pregnant had a 42 per cent lower risk of dying compared with breast cancer survivors who did not get pregnant.
"Our findings clearly demonstrate that pregnancy is safe in women with history of successfully treated breast cancer," Azim said in a statement.
He added that the information is important because more young women are being cured of breast cancer, and many are delaying pregnancy to later in life.
"So it is important to provide high level of evidence to help physicians in counselling these patients. This work may result in improving the quality of life of millions of young women who finish their adjuvant breast cancer therapy and want to get pregnant," he said.
Azim suspects the relationship between hormones and breast cancer is more complicated than doctors think. While estrogen is known to trigger breast cancer, very high doses of the hormone can also kill cancer cells through a process called apoptosis, or programmed cell death, Azim said.
Other hormones are elevated in pregnancy, such as the key hormone involved in breastfeeding, prolactin.
"There is evidence suggesting that women with high levels of prolactin have a reduced risk of breast cancer relapse," he said.
Azim added that there could be immune responses at play as well.
"It has been shown that fetal antigens are expressed on the tumour cells of the mother. Thus, antibodies produced by the mother in response to these antigens, may act as a kind of tumour vaccination," he hypothesized.
He said another reason for the findings might be that women who were naturally healthier were those that later had children.
Azim says his team is now refining the study to examine the effect of the timing of pregnancy on the risk for cancer recurrence. They want to know, for instance, how soon after a breast cancer diagnosis it is safe to become pregnant, or if there are differences in survival according to the patient's age, lymph node status and so on.
