Young women who have had an abortion may be at increased risk of developing mental health problems, a Christchurch study suggests.

The study, the largest of its type internationally, poses challenges to the grounds on which abortions are granted in New Zealand and some other countries.

The leader of the Canterbury Health and Development study, David Fergusson, said his researchers had followed the progress of 1265 children born in Christchurch in mid-1977 from infancy to adulthood.

Its latest research found 41 per cent of the women studied had become pregnant by age 25, and 14.6 per cent of the women studied had had an abortion.

By the age of 25, the study found, 42 per cent of those who had had an abortion had also experienced major depression during the previous four years.

This was nearly double the rate of those who had never been pregnant and 35 per cent higher than those who had chosen to continue a pregnancy.

"Those having an abortion had elevated rates of subsequent mental health problems including depression, anxiety, suicidal behaviours and substance use disorders," said the researchers, whose study has been published in the Journal of Child Psychiatry and Psychology.

Under the 1977 Contraception, Sterilisation and Abortion Act, abortion is an offence under the Crimes Act unless two "certifying consultants" approve it on certain grounds, usually that a woman's mental health would be endangered by continuing the pregnancy.

The Abortion Supervisory Committee last year repeated its call for a tightening of the law governing abortion saying that the wording of the law had come to have a "de facto liberal interpretation."

In 2004, 18,211 abortions were performed in New Zealand.

Professor Fergusson said there was little evidence to suggest abortion improved mental health.

"This study suggests possibly the opposite," he said yesterday.

"Certainly I think the pro-life groups will be beginning to argue that the evidence is against the basis of the legislation. That would be one possible implication."

"I appreciate there are people with strong ethical viewpoints about abortion and they have every entitlement to them but the extent to which they have a right to impose those views on others is where I depart and that is the foundation of my pro-choice position."

While the results of the study were only suggestive, it should lead to larger studies on such an important issue, he said.

"Abortion is by far the most common medical surgical procedure that young women receive.

"In our cohort one out of seven had had an abortion by the age of 25.

"If we were talking about an antibiotic or an asthma risk, and someone reported adverse reactions, people would be advocating further research to evaluate risk," Professor Fergusson said.

"I can see no good reason why the same rules don't apply to abortion."