"It's just never been a part of the regular regimen used in this country," says Bill Camann, an obstetric anesthesiologist at Brigham and Women's Hospital in Boston, which will offer nitrous for labor this year. "And you know, old habits die hard and people are set in their ways, and for no good reason."

The nitrous comeback has been sluggish. The reasons for that are varied, but come down to culture, turf, and money.

Though it's ubiquitous in dental offices, getting gas for labor is not like getting gas for a root canal. The machines used for labor are set to a 50/50 blend of oxygen and nitrous and can't be adjusted, whereas dental gas is delivered can be regulated and delivered in much higher concentrations, up to 70 percent. At the 50/50 level used for labor, nitrous is not an anesthetic; it's simply an analgesic.

"If a woman uses nitrous she'll say 'I feel pain,’ but, she'll say, 'I just don't really care," says Elizabeth Kester, a registered nurse who manages the Women and Children's Health Services at Monadnock Community Hospital in Peterborough, New Hampshire, which was the first hospital in New England to offer nitrous. That ambivalence, is due to the dissociative effect of the gas, Kester says.

Women using it in labor get nitrous oxide through a facemask or a mouthpiece. Just before a contraction they take a big breath in, which releases the gas. They then breathe out into the mask. And as long as they do it right, the machine then scavenges any nitrous out of that exhalation, saving the rest of the people in the room from sampling some of that heady elixir on accident.

In less than a minute, usually about 30 seconds, the woman will feel the effect of the gas, but as soon as she takes the mask off and takes a few breaths of room air, the gas clears her body.

There are plenty of reasons to favor nitrous for pain relief during labor. First, the sensation of being on nitrous falls somewhere between feeling nothing and feeling everything. It also gives the laboring woman a sense of control, since she is the one who determines when she needs the gas and when she doesn't.

"If a woman doesn’t like the way she's feeling with the gas, she can just take off the mask and it's gone," Kester says. "With narcotics or an epidural if a woman doesn't like the way it feels, she kind of out of luck."

Narcotics such as morphine or fentanyl take up to two hours to wear off and can leave a woman feeling drugged. And epidurals typically numb a woman from the waist down, restricting her mobility for several hours. Side effects from nitrous oxide can include nausea, dizziness, and drowsiness.

Nitrous is also relatively safe. Both narcotics and epidurals come with safety concerns, and epidurals have an increased risk of the mother needing medical interventions later in her labor, says Michelle Collins, associate professor and director of the nurse-midwifery program at the Vanderbilt University School of Nursing.