Pregnant women with cancer should seek treatment and should not terminate the pregnancy over concerns of whether cancer therapies may harm their unborn child, researchers say.

The finding comes from a study involving 129 European children ages one to three born after a prenatal exposure to their mothers' cancer treatments, they say.

The development of those children's heart function and mental processes was normal compared with an age-matched group of other children in the population at large, the researchers have reported in the New England Journal of Medicine.

"Our results show that fear of cancer treatment is no reason to terminate a pregnancy, that maternal treatment should not be delayed and that chemotherapy can be given," says Frédéric Amant, a gynecological oncologist at hospitals in both Belgium and the Netherlands.

Children are more likely to suffer more as a result of being born prematurely than from their mothers' chemotherapy, he adds, so avoiding a premature birth should be considered more important than choosing to avoid such cancer treatment.

Of the children born to those mothers undergoing cancer treatment, 89 were exposed to some form of chemotherapy previous to birth, four to radiotherapy, seven to both chemotherapy and radiotherapy, and 13 to surgery on the mothers.

The most common cancers among the mothers were breast cancers and blood-related cancers such as lymphoma and leukemia.

"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," says Amant. "Nor was the number of chemotherapy cycles during pregnancy, which ranged from one to 10, related to the outcome of the children."

The researchers acknowledged some limitations in their study. Although the data analyzed covered many kinds of chemotherapy, the results cannot guarantee all kinds of chemo are safe, they said.

"We need to look at larger numbers of children and larger numbers exposed to each drug in order to be able to document the potential effects of individual drugs," Amant explains.

The study will also continue to monitor and follow the children of the mothers who underwent cancer treatment until those children turn 18, he told the 2015 European Cancer Congress being held in Vienna.

"These latest results should be reassuring for pregnant women who have been diagnosed with cancer and who will, naturally, be worrying about the best course of action not only for themselves but for their unborn child," says Peter Naredi, scientific co-chair of the cancer congress, who was not directly involved in the study.

The message of the study, he said, is that doctors need to begin cancer treatment as soon as a diagnosis is made while working to continue the pregnancy for as long and as close to full term as is possible.