Babies whose mothers undergo cancer drug treatment during their pregnancy do not appear to suffer any long-term harm, according to a groundbreaking study.

When a pregnant woman is diagnosed with cancer, she and her family and doctors are faced with difficult decisions about her health and that of her unborn child. It is known that giving chemotherapy in the first 12 weeks, when the baby's organs are still forming, raises the chances of birth defects.

Postponing the treatment, however, could allow the cancer to spread. Over the last decade, doctors have been more willing to use chemotherapy after the first trimester, but there have been worries that the child's brain and heart could suffer damage.

Researchers in the Lancet Oncology journal report that they have followed the progress of 70 children whose mothers had chemotherapy while they were in the womb – and their findings are reassuring.

The study began in 2005, recruiting some children retrospectively and following others from birth. Every few years the children, who ranged in age from 18 months to 18 years, were given physical and mental development tests.

The children's thinking and reasoning capacity was similar to that of other children – but those born prematurely, often because it had been thought advisable to deliver the baby early to begin the mother's cancer treatment, had lower development scores than those who were born around term (40 weeks' gestation). Allowing for variables such as age, sex and country of birth, the child's IQ was 12 points higher for every extra month he or she spent in the womb.

Dr Frédéric Amant, from the Leuven cancer institute at the Katholieke Universiteit in Leuven, Belgium, and colleagues say in their paper that the difference was not down to the cancer treatment, but to prematurity. The same lower developmental scores are seen in all very premature babies.

One pair of twins did have a severe neuro-developmental delay, but the scientists said they did not think this was related to the chemotherapy – although follow-up of a larger group of children for a longer period of time is needed to be certain that there are no adverse effects from the cancer drugs.

On behaviour, hearing, general health and growth, the children did as well as any others. Their heart size and function was normal too.

As women have babies at a later age, the numbers of those diagnosed with cancer while pregnant is rising, the authors say. Their work suggests cancer treatment should not be delayed if a pregnant woman needs it and that it may be more damaging to the child in the long term to wait and then deliver the baby prematurely.

"We show that children who were prenatally exposed to chemotherapy do as well as other children," conclude the authors. "The decision to administer chemotherapy should follow the same guidelines as in non-pregnant patients. In practice, it is possible to administer chemotherapy from 14 weeks gestational age onwards with specific attention to prenatal care."

But they add that they cannot be sure there will be no effects on the children in years to come and more follow-up is needed to be sure the children will not have impaired fertility as adults or be more likely to get cancer themselves.

"Only time will inform us of the full consequences, including fertility and secondary malignancies (especially if DNA damaging drugs are used), of foetal exposure to chemotherapy," they say.

In an editorial in the journal, Dr Elyce Cardonick of the department of obstetrics and gynaecology at Cooper Medical School of Rowan University in New Jersey, said: "The study by Amant and colleagues has the potential to affect clinical practice: if we can present this reassuring data to pregnant women with cancer, women might be more likely to accept treatment during pregnancy when indicated."

Case study: Caroline Swain

It was the lump in her breast that Caroline Swain discovered first. Although she and her husband had been trying for a second child, when she went for the consultation that would confirm her breast cancer, she did not feel pregnant. But she did a test just in case.

"Normally you would be overjoyed, but there was this apprehension, thinking, 'Oh my goodness, what happens now?'" she says.

Termination was not something that she wanted. "There is this real kind of inner deep need to save this baby inside you, but you don't want to put your chance of survival at any greater risk. I wanted to survive for the lovely baby son I already had. It was a really weird emotional rollercoaster."

She was lucky to have a very supportive surgeon and oncologist, she adds. They were not familiar with treating pregnant women for cancer, but after doing some research, they told Swain she could be treated. They advised a radical mastectomy, removing the entire breast and lymph nodes. This would ensure that she would be exposed only once to the dangers of general anaesthetic and surgery.

The baby survived, but, she says, "it was all very sad and traumatic. I was losing my breast, which is itself something to come to terms with and faced with a diagnosis of cancer which meant I might not see my little boy grow up, and he wouldn't remember me if I did survive for five years."

She was told she could have full-dose chemotherapy after 12 weeks of pregnancy but struggled at first to understand how this would not damage her baby. "How can it be possible to have chemotherapy when you are pregnant when you are not even supposed to take a headache pill?" she says. "Finally I got my answer – that the placenta acts as a barrier to these drugs and a very minimal amount gets through."

Swain says she felt guilty about having chemotherapy, but her second son, Luke, was born at term with a normal delivery and no ill-effects and is now a very healthy nine-year-old.