Labor pain is nothing to laugh at. Yet. The use of nitrous oxide, or laughing gas, during childbirth fell out of favor in the United States decades ago, and just two hospitals -- one in San Francisco and one in Seattle -- still offer it. But interest in returning the dentist office staple to the delivery room is growing: respected hospitals including Dartmouth-Hitchcock Medical Center plan to start offering it, the federal government is reviewing it, and after a long hiatus, the equipment needed to administer it is expected to hit the market soon. Though nitrous oxide is commonly used for labor pain relief in Canada, Great Britain and other countries, it's been all but abandoned in the United States in favor of other options, such as epidurals. With an epidural, medication to block pain seeps through a tube into space surrounding the spinal cord. Because it must be administered by an anesthesiologist, an epidural is significantly more expensive than nitrous oxide. Both are covered by insurance.