In an analysis of 21 studies involving more than 3,700 women, researchers found that the first stage of labor was one hour shorter, on average, when women were mobile or at least upright rather than lying down.

There was no evidence of adverse effects on mothers or newborns.

In developed countries, it is standard practice for women to go through labor in bed, making it easier for doctors and nurses to monitor the fetus and labor progression. Epidural pain relief and IV infusions can also limit women's ability to move.

But the current findings suggest that women can safely choose to move around during the first stage of labor -- the longest stage of childbirth, during which contractions gradually become stronger and occur closer together.

"Women should be encouraged to take up whatever position they find most comfortable in the first stage of labour," write Dr. ANNE Marie Lawrence, of the Tonsilla Hospital in Douglas, Australia, and her colleagues.

Lying down, the researchers note, puts the weight of the expanded uterus on abdominal blood vessels and may weaken a woman's contractions, which could in turn make the first stage of labor longer.

The findings are based on data from 21 clinical trials in which women were randomly assigned to recline or sit up, kneel, stand or walk around during the first stage of labor.

On average, women who were upright had a shorter first stage of labor, though there were no differences among the groups during the "pushing"

stage. Women who spent time in upright positions were also somewhat less likely to have an epidural.

Future studies, Lawrence and her colleagues note, should look more closely at how childbirth positions affect the risk of complications, such as hemorrhage, and whether allowing women more freedom to move around improves their childbirth experience.