To get the fetus out, your uterus has to contract, restricting blood flow and causing pain. Though how intensely you feel this pain is subjective, there are several factors that can influence how uncomfortable labor will be:
The size of your baby and of your pelvis
The bigger the fetus and the smaller the pelvis, the harder the uterus has to contract. A large baby can also put more pressure on the nerve endings, causing more pain as he passes through.
The baby's position
Most babies travel through the birth canal face-down. One who's faceup presses against your back nerves, since the hardest part of her head is flat against your pelvic wall. (This is often referred to as back labor.)
Your physical condition
Some experts believe that women who are in good shape have easier labors, possibly because their muscles are stronger or they've built up better endurance.
Those who've delivered vaginally before have shorter labors in general -- 8 hours on average versus 14 hours for first-timers. The ligaments and muscles in the pelvis have loosened up and are less likely to provide as much resistance the second time around.
Pitocin, the drug that's used to jump-start labor, can cause some women to experience stronger contractions from the get-go (as opposed to a gradual buildup).
A woman who's completely unprepared and has no support is much more likely to experience severe pain. So talk to new moms, read about what to expect beforehand, and make sure your partner is able to give you the comfort and reassurance you need.