Health reporter, BBC News
It is thought radiotherapy affects the womb
Women who underwent radiotherapy for cancer as children are at risk of having a premature or underweight baby, a large British study shows.
Analysis of data from more than 10,000 female survivors also showed an increased risk of miscarriage in those who had radiotherapy to the abdomen.
The Birmingham University researchers said doctors should offer closer monitoring in these women.
No link was found between chemotherapy and pregnancy problems.
The study - part of the ongoing British Childhood Cancer Survivor Study - is the largest of its kind, and looked at data from 7,300 pregnancies in 31% of the survivors.
It showed a three-fold increased risk of delivering early and a two-fold increased risk of having a low birth-weight baby.
It is not exactly clear why the risk is increased but the theory is that the treatment leads to a smaller womb and reduced blood flow to the womb.
The study also showed a 40% increased risk of miscarriage in female childhood cancer survivors, the journal Cancer Epidemiology, Biomarkers and Prevention showed.
But there was no increased risk of miscarriage with brain radiotherapy, which the researchers said had been suggested in the past.
Overall female survivors of child cancer had a third fewer children than the general population.
And those who underwent radiotherapy had half the number of children than the general population.
The study did not look at why this may be, but the researchers said reasons could include that women undergo early menopause because of their treatment, find it harder to find a partner, and are concerned about the health effects of their treatment on their children.
Study leader Raoul Reulen said major improvements in treatment for childhood cancer meant many were surviving into adulthood and so long-term effects were becoming more important.
He also pointed out that some patients become infertile because of their treatment but many remain fertile and would like to have children.
"For women in this situation wanting to become pregnant, it is important to look at what kind of treatment they received in the past and to talk about that with their doctor.
"If they had high-dose abdominal radiation and want to be pregnant they should be referred to a specialist.
"But for other survivors, such as those who had chemotherapy, it's reassuring because there is no evidence of adverse pregnancy outcome."
Kate Law, director of clinical research at Cancer Research UK, said it was important to remember that the treatments looked at in the study meant that seven in 10 childhood cancer patients survive and therapies were improving all the time.
"If a childhood cancer patient is concerned that they may be affected by this and is planning to have a child she should contact her doctor who will be able to give them the appropriate advice."