According to an article in the Boulder Daily Camera ("Laughter really may be among the best medicines," Nov. 4, 1996), laughter helps control pain, lower blood pressure, relieve stress, and increase muscle flexion. It also shuts off the flow of stress hormones, increases the body's ability to utilize oxygen, and triggers a flood of beta endorphins, the brain's natural morphine-like compounds that can induce a sense of euphoria. Although the article doesn't specifically mention birth, the following comments from birthing women, midwives, and doctors speak for themselves:

"Even though I was still on my hands and knees, my hearing suddenly became very acute. I could hear Gordon on the phone in the next room: Glenn? This is Gord. Could you ask Elly to come over. I think the baby's coming. You think the baby's coming? I echoed to myself. And suddenly, I laughed. I could not help it - the man's hesitation struck me as funny. I laughed at the ridiculousness of it all. Suddenly, I was looking down a tunnel the long way around, as if a telescope inside me - that was somehow outside me - was turned backwards. As I laughed, the baby's head popped out. I tightened my pelvic floor muscles and, turning my head, noticed Gordon at the doorway. Imagining how ludicrous I must have looked, reared up on my haunches with a baby's head sticking out of me, I laughed again. This time, the baby simply fell out into Gordon's out-stretched hands."
-From "They Don't Call it a Peak Experience for Nothing," by Ruth Claire (Mothering, Fall 1989)

"Then Dick said, 'The head's showing!' I laughed and said I knew. I gave one firm push while pressing on my clitoris like it shows in Birth and the Dialogue of Love (page 126) and it really helped. (It was a big head.) I wanted to lay down and then I did. Dick was still hugging me and I was laying on him and laughing inside at what a sight it must have been. He told me to let him up. I didn't want to move but I did. He got out from under me, grabbed some clean towels, and put them on the floor. I knelt and he caught the baby all in a matter of seconds."
-From The New Nativity

"She danced and danced throughout her labor. No noise, no fuss, just intense concentration and dancing. After many hours she looked up at me with a puzzled expression and said: 'I can't do this anymore.' I asked if I could check her, and when she opened her legs, the baby's head was crowning. I told her that was why she felt that way, and she began to laugh, and laughing, birthed the baby into my surprised hands. Her partner took pictures: a baby born en caul to a laughing VBAC (vaginal birth after cesarean) mother. We must witness, talk story, tell the good stories to counteract our culture's horror of birth. It is possible to birth in peace and joy."
-Anne Stohrer, M.D. in Compleat Mother magazine, Winter, 2000

"I noticed that whenever Judith would laugh at something, she'd have a very good rush [contraction] right afterward, which would dilate her cervix a bunch more. So we all sat around and had a good time talking with each other, and after a few more rushes I checked Judith again and found that she was fully dilated and ready to push the baby out."
-From Spiritual Midwifery, by Ina May Gaskin

"Sarah announced her desire to enter the world quite amusingly. As Rich looked down to see where we were, the waters broke in his face...We laughed until the next contraction which brought the crowning of her head. Then two more contraction-pushes and she was here!"
-From Two Attune

"I sat down on the floor and leaned against the pillows. The desire to push overcame me. For several pushes I pushed with caution while Bob rubbed ointment into the perineal area. He informed me that he would 'do the looking.' I had to laugh at Bob's comments. I began to pray this this would be the last push and began to act on that request with all the power within me. At this point Bob used his baseball training because our Naomi became a pop fly. Once again I found myself laughing at Bob's crazy monologue. Such sweetness!"
-From The New Nativity

"I was laughing when I was pushing, and he was just in awe. And when he came over holding this tiny baby, it was probably the most beautiful moment of my life."
-Melanie Griffith (Mrs. Antonio Banderas) in Ladies Home Journal, Sept., 1997

"Brian and I were smiling and even laughing in between contractions because even though they were very intense I felt so good in between them; it was like I wasn't even in labor. I had one really big contraction and my water broke. I felt like pushing so I got in a comfortable sitting position and with the next contraction my baby's head was out. One more push and our son was in Brian's hands."
-From The New Nativity

"With one arm around Shara and another around Cary, we attempted to walk, push, walk, push. 'Are you giggling?' Cary asked. 'No,' I smiled, I was laughing. Although my bottom was bursting, my clitoris was tickling. It felt almost orgasmic. So all of us walked around the room laughing. What a sight."
-From "Homebirth Minus a Midwife" by Alaina Chapman (Compleat Mother, Summer 1994)

"We returned to the bedroom where I positioned myself on the end of the bed to be in position for the baby to be born. (Our bed is on the floor.) I was half-sitting, half-squatting and pushing hard. The bag of waters broke then, gushing all over Bill, who was on the floor in front of me and we laughed. Oh, that felt so good."
-From The New Nativity

"Mary Louise came. She was getting stuff together and I was rushing and started noticing that when I looked in her eyes through a rush I got some strength to feel it as a force that was intelligent and courageous. I noticed that when I looked at Edward through one I felt it as a pain. When I asked her about that, she said that it was because she wasn't believing that it was painful and that I needed to keep my sense of humor and be nice to Edward. That clicked and with the next rush I laughed, and started laughing as they came. That got the energy up higher and of course the rushes came on stronger."
-From Spiritual Midwifery, by Ina May Gaskin

"I caught Max myself in a bathtub of lovely, relaxing warm water - and he was born in a 'laugh,' as I was vocalizing a HO - HO - HO sound (a very open sound which came instinctually)."
-From Waterbirth! by Cat Majors

"I didn't think the baby was moving down very fast. Loring crawled back up to tell me I was doing fine. Then he gave me a long kiss and crawled back to inspect my bottom. On his next trip back up to smooch with me he told me I had a cute behind and I really had a laugh. I guess the laugh was what I needed to do."
-From The New Nativity

"I was afraid to laugh because I thought it would make the baby come out. This was true and I realized that if I laughed and loosened up that the pain would go away and Louisa would come out really easy and everything would be psychedelic and Holy. I laughed. Mark and I started smooching a lot to keep my mouth really loose. This made the contractions come on really strong."
-From Spiritual Midwifery, by Ina May Gaskin

"Even a forced smile releases endorphins, the body's natural pain medicine that is similar to morphine. When we are with a birthing woman who is in pain, it may help to tell some good jokes - or even some not so good ones, especially in early labor. Carry a joke book in your birth bag!"
-Marianne Manely, in Midwifery Today Issue 47

"Not only is humor good for the mind, it's good for the body, especially in labor. Laughter increases the level of endorphins, the body's natural pain relievers and relaxers. It decreases levels of unwelcome stress hormones and relaxes muscles. Laughter is like an internal massage. As the ancient writings of Proverbs advise, 'A cheerful spirit is health to the body and a strength to the soul.' Bring a bit of humor to your birth."
-From The Birth Book by William Sears, M.D. and Martha Sears, R.N.

Midwife Ina May Gaskin on laughter in labor:

A mother laughs as she births her baby into her own hands.

She continues to smile as her baby emerges.

A triumphant mother holds her newborn baby.