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By Maggie Fox

It’s standard advice in so many delivery rooms: don’t push until it’s time.

Usually, mothers giving birth are told to wait until the midwife, doctor or delivery nurse says it's time — usually about an hour after the cervix is dilated to a certain diameter, often 10 centimeters.

Even the bible for many pregnant women — the book and website “What to Expect When You’re Expecting” —suggests that women need to wait to push.

The common wisdom was that waiting made the delivery easier and safer.

But a new study turns that idea on its head. It turns out that, after the cervix dilates to 10 centimeters, it makes no difference whether women wait to push or just go ahead when they feel ready.

“Both approaches are commonly used, and neither is considered the gold standard,” Dr. Alison Cahill of Washington University in St. Louis and colleagues wrote in their report.

But when women start pushing earlier, the total time to delivery tends to be shorter, the researchers report in the Journal of the American Medical Association.

Many doctors moved to the delayed-pushing approach when one big study done in the last century showed that delaying the push reduced the risk that the obstetrician would have to use forceps to help get the baby out.

“Deliveries using mid-pelvic forceps were prevalent 20 years ago when that study was conducted; however, these types of deliveries are obsolete in modern obstetric practice in the United States,” Cahill and colleagues wrote.

Many doctors also believed that if women delayed pushing, the need for a caesarean section would be lower.

The study of 2,400 women giving birth for the first time showed that delaying pushing did not reduce the rate of a c-section. The women all had epidurals and were randomly assigned to either delay pushing, or to go ahead and push when they felt like it, once they'd reached 10 cm dliation.

In both groups, about 85 percent of the babies were delivered without surgery. There was no significant difference in the risk for hemorrhage between the groups.

But the rates of some rare complications were lower in the group of women who were allowed to push when they pleased. Babies were less likely to suffer high acid levels in their blood, which is a complication of going without oxygen for too long. They were also less likely to develop chorioamnionitis, a complication caused when bacteria are forced up into the uterus during prolonged labor.

And overall time to birth was on average 30 minutes shorter when women were allowed to push earlier, the researchers found.

“The findings provide strong evidence that for the vast majority of first-time mothers receiving epidural anesthesia, delaying pushing offers no benefit over immediate pushing at the second stage of labor,” said Dr. Menachem Miodovnik, of the National Institute of Child Health and Human Development, which helped pay for the study.

It’s good to have some hard evidence to guide doctors and delivery room staff, said Dr. Jeffrey Sperling and Dr. Dana Gossett of the University of California, San Francisco, who were not involved in the study.

“Almost 4 million births occur in the United States each year, of which approximately 68 percent involve vaginal delivery, so additional evidence to guide the management of labor is critically important,” they wrote in a commentary.

There is probably no specific time at which it’s better to start pushing, they said.

Now, they said, it will be important to do a similar study among women giving birth for the second or third time.