Nitrous oxide or entonox as it is commonly called is used in labour to help with the pain of the contractions. The woman controls the apparatus and inhales the gas through a mouth piece at the start of the contraction and when the contraction has finished, takes the mouth piece away. A lot of women find entonox very helpful in the relief of pain, while others do not like its taste or its effects.
"This is one of those news stories where my biases show, like woah, and I'm happy to admit it. In the UK, 50% of laboring women take advantage of nitrous oxide (good ol' dentist's office laughing gas) for pain relief. The rates of usage are about the same in Canada and even higher in countries like Finland. If you read up on the stuff, this isn't terribly surprising. Nitrous oxide gas doesn't totally eliminate childbirth pain, but studies show it does a great job of taking the edge off and without the long-term loopiness of injected narcotics or the limitations on movement (and the whole needle-in-your-spine bit) that goes along with epidurals. Plus, it's pain relief that's controlled by the woman, herself. You just take a hit off the gas whenever you find your contraction warrants it. No anesthesiologist necessary."
This article appeared in Boing-Boing. Unfortunately the link to the page where it explains why entonox is not commonly used in the US is not working. I did some research of my own to find out why and apparently it did not catch on in the US as it did in Europe and over here in New Zealand, also there appears to be insufficient evidence as to the effectiveness of the analgesia and the affects on the fetus. The American College of Nurse Midwives recognizes the use of nitrous oxide in labour and its safe administration.
Nitrous Oxide use in the US