Health reporter, BBC News
Being overdue is a common reason for induction
Concerns have been raised about pregnant women being induced "unnecessarily", after a Scottish audit of 17,000 births.
In more than a quarter of cases, researchers could not find a medical or other explanation for the procedure.
The Aberdeen University team said in the Journal of Public Health that rates of obstetric intervention were rising.
Experts warned that more studies were needed to assess whether inductions are being done without good reason.
In the UK, the commonest method of induction is the use of a gel containing prostaglandins to bring on contractions and start the labour process.
Medical reasons for induction include being over 41 weeks pregnant, waters breaking but no onset of labour, or planned timing of labour because of complications.
The procedure is associated with an increased likelihood of further medical interventions, such as caesarean section.
Rates of induction are rising in the UK - particularly in Scotland - but there does not seem to be a single explanation for the figures, the researchers said.
Using data in the Aberdeen Maternity and Neonatal databank on births between 1999 and 2003, they identified that 5,700 or 32% of pregnant women had been induced.
Reasons for induction included the standard medical reasons as well as social factors such as living a long way from the hospital.
But 28% of cases remained unexplained.
"I did expect to find a proportion that could not be explained because as good as the database is I'm sure there are characteristics that are just not collected, but we were slightly surprised that it was as high as it was," said study leader Tracy Humphrey, consultant midwife at Grampian NHS Board.
"It raises the question that there may be some unnecessary interventions and we are planning further studies."
She added: "The main debate about unnecessary interventions has focused on caesareans and we have forgotten about the other interventions that go on."
Mary Newburn, head of policy research at the National Childbirth Trust, said that if a significant number of inductions were being done without cause, it was a serious problem which must be addressed.
"Further research is needed to identify the circumstances of induced labour where there are not good clinical reasons."
She added that induced labour tends to be more painful and more difficult for women to manage without strong drugs.
"If women are asking for induction because they are tired at the end of pregnancy, they may need more support, and encouragement to rest as much as possible, so that they can wait a bit longer."
Patrick O' Brien, a consultant at University College London Hospitals and a spokesperson for the Royal College of Obstetricians and Gynaecologists, said the finding was interesting but it may be that reasons for induction were not properly recorded in the database.
"But it is important that there is a good reason for inducing labour when it happens," he added. "More research needs to be done."
Sue Macdonald, spokeswoman for the Royal College of Midwives, said it was important that women knew why induction was being done.
"We are very interested in trying to support normal birth and ensuring an intervention is done only when it's absolutely necessary."