However, a premature birth may reduce the risk of developing the disease in the short-term.

Breast cancer is known to be influenced by various hormonal changes, which can increase or decrease the risk of developing the disease.

Researchers have established that the birth of a baby has a dual effect on the risk of developing breast cancer.

First, there is a short-term increase in risk, which is followed in the longer term by protection against the disease.

Major study

An international team of scientists, writing in the medical journal The Lancet, report the results of a study of 807,874 Swedish women who had babies born after a gestation period of at least 28 weeks from 1973 to 1989.

Women diagnosed with breast cancer during the same period were also studied, and information on their childbirth history was analysed.

The researchers wrote that "premature delivery was associated with a slightly lowered breast cancer risk within five years after the delivery compared with term dilivery, whereas it was associated with increased risk later on".

The reason could be that, unlike a full-term pregnancy, premature deliveries are not accompanied by a surge in hormone production late in pregnancy.

The researchers call for further work to study the effect of different pregnancy outcomes on maternal breast cancer risk.

Dr Chung-Cheng Hsieh, of the University of Massachusetts Cancer Centre, said: "The very high levels of oestrogen associated with the late period of gestation may stimulate cells that have already begun a malignant transformation to grow further.

"This effect is less evident in premature births because the ostrogen surge does not take place.

"However, a full-term pregnancy also helps to induce more cells to develop fully into mammary cells. It is the undifferentiated cells that are most likely to become malignant, and so this effect reduces the possibility of developing breast cancer in the longer term.