Pain in labor comes from the following sources
As you can see, pain in labor may come from a number of different sources, and every woman experiences them differently. You may be surprised to see the last two listed as major sources of pain, but uncomfortable positions (mainly, being confined to bed, especially on your back) and fear both increase pain in labor. But I'm getting ahead of myself...
Muscle fatigue in the uterus
One of the main sources is the uterine contractions themselves. Contractions usually start off as painless. You're likely to start having them in the second half of your pregnancy, and it feels as though your womb was just replaced by a basketball. These painless (though perhaps mildly uncomfortable) contractions are called Braxton-Hicks, and you can think of them as your uterus exercising and getting ready for the big day. Braxton-Hicks often come with increasing frequency as you get closer to giving birth. When you go into labor, and as you move through the stages of labor, your uterus will continue to contract with increasing frequency. What's painful about that?
The first thing is that the uterus keeps contracting and relaxing. If you've ever tried the trick where you lift a blade of grass above your head 200 times, you know that after about 20, your arm feels like it's about to fall off. After about 40, it's a serious ache. The same thing happens to your uterus. As it keeps contracting, this itself becomes painful because the muscle gets tired. At term, your uterus is the strongest and biggest muscle in your body. When it gets tired, you really feel it.
Pressure/stretching on the cervix, the pelvic bones, the pelvic floor and the perineum
The other thing that's painful about effacement and dilation is that the cervix is sensitive to pressure and stretching. As the baby's head exerts pressure on the cervix, this triggers more contractions, which pushes the baby's head deeper into the cervix, which puts more pressure on it and stretches it more, which produces more contractions... you see the pattern. This is called a positive feedback loop, and it's what keeps labor going stronger and stronger until the baby is born
The point of this process is to open the cervix to allow the baby to descend into the birth canal and out the body. The uterus is a bag of muscle. Imagine an upside-down turtleneck sweater, where the turtleneck is the cervix and the bottom of the sweater is actually sewn shut. Contractions originate at the top of this upside-down turtleneck sweater (near the baby's bottom), and act to shorten and then open the turtleneck down below. Like any muscle, when the uterus contracts, it tightens and shortens. The thing that's different about the uterus is that once it contracts in labor, relaxing again does not lengthen the muscle fibers. Instead, these muscles stay shortened. In this manner, the bag of muscle slowly pulls the cervix up and apart. This is called effacement and dilation, and it is the first stage of labor.
Once effacement and dilation is complete, the gates are now open and the baby is free to descend into the birth canal. This means, contractions are now not just opening the cervix, but pushing the baby out. The baby begins to maneuver through the bones of the pelvis, shimmying and turning to find the best fit through this crooked canal. Along the way, the baby's head pushes on the bones of the pelvis, stretches the tissues and joints, and puts a lot of pressure on the pelvic floor, the rectum, and the perineum (the stretchy tissue between the vaginal opening and the anus). This is the pushing stage of labor, also known as second stage.
As you can imagine, with all this pressure here and pushing there, if you're confined to a single position, it can become cramped and uncomfortable. Women instinctively move in labor to find the position that is least painful. This is RARELY on their back. Every woman I've ever seen in labor couldn't wait to get off her back, and those that were confined to a single position were very vocal about how much worse that made bearing the contractions.
Fear is another huge source of pain. For a couple of reasons. The first is that when you are afraid, you tense up your muscles. You're actually working against yourself! Your uterus hurts because it is repeatedly contracting and relaxing. It is achy from all that work. And fear tenses you up and makes it even harder for the uterus to do its job, which means it gets tired faster, and it hurts more. Another reason fear is bad is that it keeps you in "fight or flight" mode, and prevents your own natural painkillers - endorphins - from kicking in. Fear also makes you focus on the pain, which makes you experience it more. You know how you sometimes discover quite painful-looking bruises on your body and have no recollection of how you got them? But when you anticipate being hurt, you feel it all the more? This is the same thing.
What does labor pain feel like?
Labor pain usually begins as some menstrual cramping, felt mostly in your lower belly or in your back, and builds up to where it will encompass your whole midriff. Some women also compare the type of pain to having a charley horse or gas pains or waves of diarrhea cramps. The intensity goes from mild to surprisingly unpleasant to mind-altering. Your job through the first stage of labor - dilation and effacement - is to find a way to allow these contractions to do their work and open your cervix. And altering your mind is really one of the best ways to deal with the intensity of labor pain. Entering into a zen state of altered consciousness allows women to handle the intense contractions of active labor.
In the second stage - pushing - the baby's head begins to move down the birth canal, and put pressure on the bones and tissues in the area. While still having contractions, many women find pushing to be a relief. Instead of merely enduring, you can now actively participate! Gone is the floaty altered state of consciousness; most women are extremely clear and focused during pushing. You may experience the baby's descent as pain in your hips, sacrum, pubic bone. The baby's head will also be putting pressure on your rectum, which feels like like having a bowel movement. In fact, when you feel like you have to poop, that's excellent news! The baby's head is coming. Women describe the feeling of pushing the baby out as tremendous pressure, constipation, a feeling of being torn apart. When the largest part of the baby's head is stretching your vagina, known as crowning, many women describe this feeling as a ring of fire.
All this time, while you are working hard to allow the contractions to proceed unhindered, and then when you are working with your contractions to push the baby out, your body is producing massive amounts of hormones that were designed to help you bear the pain and make you feel good. As soon as the baby is out, the pain is gone, but the massive amounts of happiness-hormones are still there. If you are a runner, you know what I'm talking about. This is like the best runner's high times ten.