Medically inducing labour does not increase a woman's risk of a caesarean delivery as thought, U.S. researchers say.

In Tuesday's issue of the Annals of Internal Medicine, researchers reviewed randomized trials that assigned women to an elective induction or to wait until late. They concluded elective induction of labour at or after 41 weeks of gestation lowered caesarean risk by 22 per cent compared to waiting.

The take-away message for pregnant women is that induction can be done without increasing the risk of making C-section necessary if obstetricians are willing to give induction enough time to work, said study author Dr. Douglas Owens, director of the Stanford-University of California, San Francisco Evidence-based Practice Center.

The rate of elective inductions has more than doubled since 1990 in the U.S., leading some physicians to worry the procedure exposed women to a higher risk of cesarean and medical complications after a surgical delivery, the researchers said.

Observational studies often compared electively induced labour at a particular gestational age with spontaneous labour at the same point in pregnancy.

Such comparisons are misleading, however, since women and their doctors can't decide to start spontaneous labour on a particular date. They can only induce or wait, after weighing the risks of induction with the risks of staying pregnant, Owens said.

Risks to fetus

Near the end of gestation, as the fetus gets bigger, staying pregnant increases a woman's chance of needing a C-section.

After the full gestational period of 40 weeks, the placenta may transmit oxygen to the fetus less efficiently, and there may be an increased need to deliver by C-section to prevent fetal distress, the researchers said.

The team also observed that women whose labour was electively induced were half as likely to have meconium-stained amniotic fluid, a sign the fetus was stressed.

The California team noted much of the research was done at academic medical centres, which raises questions about whether the findings translate for most obstetric care.

Last year, the Society of Obstetricians and Gynecologists of Canada said more than one in four babies born in Canada are C-sections, which increases risks for women and burdens the health-care system.

Between 1993 and 2006, Canada's C-section rate grew to 26.3 per cent of all deliveries from 17.6 per cent. The increase is attributed to an increase in the number of obese mothers, in which more pregnancy complications necessitating a C-section can arise, and older maternal ages.

The research was funded was by the Agency for Healthcare Research and Quality.