Dr Hatem Azim, a medical oncologist at the Jules Bordet Institute in Brussels, Belgium followed 333 women who became pregnant after being diagnosed with breast cancer and matched them to 874 women with similar cancers who did not become pregnant.
They were followed up for almost five years after the pregnancy.
Dr Azim said: “Out of all the women, 57 per cent had ER+ disease, but the study showed there was no difference in the length of time women with either ER+ or ER negative [ER-] disease survived without their disease recurring compared with those who did not become pregnant.
"In addition, we found that patients who became pregnant within two years of breast cancer diagnosis appeared to have a better disease-free survival compared to those who did not become pregnant.
"However, a clear trend over time was not demonstrated; hence this finding should be interpreted with caution as it could be confounded by potential selection bias, and hence pregnancy within two years of diagnosis should be regarded as safe, and not as protective.
“A secondary objective in this study was to determine the impact of pregnancy on overall survival. We found that breast cancer patients who became pregnant also had a lower risk of death compared to their matched controls, irrespective of ER status.”
Women who were pregnant after their diagnosis were 25 per cent less likely to die within five years than those who did not get pregnant, however this result as less clear and women should not be counselled that pregnancy is protective, Dr Azim said.
Dr Azim said: “Frequently when women with history of breast cancer become pregnant, some physicians advise them to have an abortion for fear that completing the pregnancy could have a detrimental effect on the outcome of their disease.
"We found that this was not true and the outcome was similar, irrespective of whether the pregnancy was completed or not.
"This was also the case both for women with ER+ and ER- disease. Hence, abortion should not be promoted for therapeutic reasons in these patients."
He concluded that the study 'shows convincingly that pregnancy any time following breast cancer diagnosis is safe'.
Professor David Cameron, from the University of Edinburgh, and chairman of the conference said: “This is an important study, as it can give women much more confidence that a wanted pregnancy after treatment for breast cancer does not necessarily mean a poorer chance of being able to live to bring up children.
"However, it is important to acknowledge that there are limitations in this kind of case control study, and so whilst an important piece of research, it cannot yet be taken as definitive proof that there is no adverse effect of a subsequent pregnancy. What it does suggest is that at worst any negative impact is small, and that it is likely that there is no detrimental effect of a subsequent pregnancy.”