A huge jump in first-time mothers having their labour induced is a worrying trend that is putting women at unnecessary risk, experts warn.
It is part of a broader shift towards inducing labour and is not explained by medical need, a study of more than 1.5 million births has found.
The proportion of labour induction among women who carried their babies to term rose from 18.6 per cent of all births in 1990, to 26.2 per cent in 2008.
The proportion of first-time mothers having inductions nearly doubled in that period.
By 2008 almost half of all inductions were performed on first-time mothers - and the majority of those were carried out before the 41st week of pregnancy.
But, at the same time, the rate of stillbirth remained steady.
"Induction seems to be increasing and that doesn't seem to be improving the outcomes for babies,'' study co-author Jane Ford said.
More older mothers and increases in instances of medical conditions, such as gestational diabetes, explained only some of the rise, she said. Women who were more likely to undergo induction also tended to be older, more well-off and to have private health insurance.
A leading Sydney obstetrician, Michael Nicholl, said the study was worrying because it showed inductions were often not done for conventional reasons such as high blood pressure or prolonged pregnancy.
''Unspecified'' reasons were given for between a third and half of all inductions, depending on the woman's age.
"What benefit is being achieved for either the doctor or the patient?" he said.
Despite an increasing rate of medical interventions in childbirth, little was known about why they were being undertaken, he said. But research into the increase in caesarean sections showed the overall number of women actually requesting them was low.
The study also found that as the rates of induction had gone up, so had the rate of caesareans. Between 2001 and 2007, fewer than half of the women who had their labour induced went on to have a natural birth, with a third of inductions resulting in caesareans.
Hannah Dahlen, the spokeswoman for the Australian College of Midwives, said women were often put under subtle pressure by doctors to undergo inductions and did not have the risks of further interventions explained to them.
"If you knew your chances of having a normal birth were less than half, you would think more carefully about what is being advised," she said. "They trust us and they trust the advice that they are getting is correct and that is very concerning."
She was particularly worried about the long-term consequences for women who had more than one child, as well as the number of inductions being performed before 41 weeks.
Women who had a caesarean for their first birth were more likely to have the procedure for subsequent births, leading to increased risks of complications such as the placenta growing through the uterus wall.
Last year the NSW Health Department released a policy aimed at reducing the rate of caesareans. Changing attitudes towards inductions could go some way towards achieving that goal, Dr Ford wrote in the Australian and New Zealand Journal of Obstetrics and Gynaecology.