Women who have been treated for breast cancer can choose to become pregnant and have babies, without fears that pregnancy could put them at higher risk of dying from their cancer, according to a major, new study.

In a meta-analysis of 14 trials, presented on Friday at the seventh European Breast Cancer Conference (EBCC7) in Barcelona, Spain, researchers from Belgium and Italy found that, not only was pregnancy safe for breast cancer survivors, but, in fact, it could improve their chances of survival.

Breast cancer is the most common cancer for women during their childbearing years. As women delay starting a family until they are older, and the survival from breast cancer has improved, increasing numbers of breast cancer survivors want to have babies after their cancer treatment has finished. Until now, it was unclear whether it was safe for them to do so, due to concerns that the hormonal changes associated with pregnancy, in particular the increase in oestrogen, could prompt the cancer to recur or become more aggressive.

Dr Hatem A. Azim, Jr., a Fellow at the Department of Medical Oncology at the Institute Jules Bordet, Brussels, Belgium, and colleagues in Italy analysed results from 14 trials that had taken place between 1970 and 2009, involving 1,417 pregnant women with a history of breast cancer and 18,059 women with a history of breast cancer who were not pregnant.

They found that patients who became pregnant following a diagnosis of breast cancer had a significant reduction of 42% in the risk of death compared to breast cancer survivors who did not get pregnant.

Dr Azim said: "Our findings clearly demonstrate that pregnancy is safe in women with history of successfully treated breast cancer. There is a wide perception in the oncology community that women with history of breast cancer should not get pregnant for fear of pregnancy increasing the risk of recurrence by means of hormonal stimulation. This meta-analysis strongly argues against this notion.

"Now we are refining the results by analysing subgroups to examine the effect of the timing of pregnancy – for instance how soon after a breast cancer diagnosis is it safe to become pregnant – and differences in survival according to the patient’s age, lymph node status and so on.

"It is still common that patients are faced with incorrect counselling regarding pregnancy and the chances of future fertility following the end of breast cancer treatment and, thus, they are denied the chance of getting pregnant. Nowadays, there is a rising trend of delaying pregnancy to later in life and more young women are cured from breast cancer. 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."

Dr Azim said there might be a number of explanations related to hormones or the immune system as to why pregnancy seemed to confer a protective effect on breast cancer survivors.

"It is well known that oestrogen is associated with breast cancer development. However, beyond a certain level, oestrogen exerts inhibitory effects on breast cancer cells. Laboratory experiments have shown that oestrogen and progesterone receptors located on 60-70% of breast tumours undergo apoptosis (programmed cell death) when exposed to high levels of oestrogen, which possibly mimics levels encountered during pregnancy. Furthermore, prolactin is elevated in pregnancy and there is evidence suggesting that women with high levels of prolactin have a reduced risk of breast cancer relapse." He added: "Nevertheless, hormonal changes during pregnancy are very complex, and the effect seen in this study could possibly be the result of the interaction of the different hormones rather than an action of a particular one by itself.

"Immunological theories could partially explain the possible protective value of pregnancy as well. It has been shown that foetal 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."

Dr Azim said that they had contacted all the authors of trials published after 1995 to get extra information on subgroups, and this would inform their further research.

He concluded: "Nowadays, fertility after cancer has become a top issue not only for patients, but also physicians. In 2006, the American Society of Clinical Oncology published guidelines that state that fertility issues should be discussed with patients before treatment – a recommendation we believe is of great importance."

Source: 7th European Breast Cancer Conference