Women who are diagnosed with cancer while they are pregnant can start treatment for the disease immediately and do not need to worry about the effects this may have on their unborn child, a new study has found.

European researchers looked at 129 children aged between one and three years, whose mothers had undergone treatment for cancer while they were still in the womb. These were compared with a similar number of children who had not been exposed to cancer treatment.

The children's general health and mental development were assessed at 18 months and again at three years. At the age of three, almost 50 of the children also had the functioning of their hearts checked.

The study found that those born after being exposed to cancer treatment in the womb displayed normal development, including their mental health and heart health, compared to the children who had not been exposed.

The most common cancers seen in their mothers were breast and haematological (blood) cancers, such as lymphoma and leukaemia.

Among the children, 69% were exposed to chemotherapy before birth, 3% to radiotherapy and 5% to both chemotherapy and radiotherapy. Other treatments included drug treatments and surgery.

Just over 10% of the women did not have any treatment for their cancer while pregnant.

"Compared to the control group of children, we found no significant differences in mental development among children exposed to chemotherapy, radiotherapy, surgery alone or no treatment. Nor was the number of chemotherapy cycles during pregnancy, which ranged from one to 10, related to the outcome of the children," explained Prof Frederic Amant, a gynaecological oncologist at the University Hospitals Leuven in Belgium.

He insisted that fear of treatment ‘is no reason to terminate a pregnancy' or to delay treatment.

The study did note that children who were born prematurely were more likely to have ‘delayed development of mental processes', and premature birth was more frequent among children born to mothers with cancer.

"In most cases, they were born prematurely due to a medical decision to induce preterm so as to continue cancer treatment after the delivery...However, the study shows that children suffer more from prematurity than from chemotherapy, so avoiding prematurity is more important than avoiding chemotherapy," Prof Amant commented.

He previously carried out a similar study on 70 children in 2011, however, that study did not have a control group to compare the children with. He described these latest results as ‘more robust', although he acknowledged that ‘we need to look at larger numbers of children'.

He and his team will continue to monitor the children in this study until they are 18 years.

Details of these findings are published in the New England Journal of Medicine and were presented as part of a special session on cancer in pregnancy at the 2015 European Cancer Congress in Austria.