Breast cancer in pregnancy is uncommon. As a result of this, the relatively small number of women in this situation has made it difficult to produce absolutely reliable statistics.

In recent years, however, a number of studies have been done comparing women whose breast cancers have been discovered during pregnancy with other women of a similar age who have developed the condition but were not pregnant.

Before these studies were done there was a general belief among specialists that breast cancers discovered during pregnancy carried a worse outlook than normal. What the recent results have shown is that it does seem that women who are found to have a breast cancer while they are pregnant do not seem to do so well but this is not because their cancers are more aggressive but because, on average, they tend to be more advanced at the time they are diagnosed than the cancers discovered in women of as similar age who are not pregnant.

This means that for a women who is pregnant and found to have a small breast cancer with no sign of spread, her chances of successful treatment and cure will be just as good as those of a woman who is not pregnant with a similar cancer. In the same way, if the cancer is a little more advanced, with evidence of spread to the nearby lymph nodes, the outcome of treatment will be the same whether the woman is pregnant or not.

But, during pregnancy the chances of the tumour being 'early' and very small when it is discovered are less likely. The most likely reason for this is that there is a delay in making the diagnosis. This is partly because the breasts enlarge (often doubling in volume) during the course of a pregnancy and this increase in size, together with other changes in the breast, tends to hide any signs of breast cancer. There is certainly good evidence from the studies that it takes longer to make a diagnosis of breast cancer in women who are pregnant and this extra time does seem sufficient to explain why the cancers are often at a later stage when they are finally confirmed.

So, overall, pregnancy does not make the breast cancer behave any worse, or any better, than it would have done if the woman was not pregnant and the results of treatment can still be very successful.