Like the other poster said, it takes a while for your body to become ready to push the baby out. It seems that after you have grown a baby in there for 9 months that your body would be ready for it, but at the time of birth your ceverx must dialate and thin out.
According to my doctor I had been in labor for a while. I remember when my back started to hurt, but stupid me thought that it was just back pain from the baby. It didn't occur to me that I was in labor. It was friday night that I started to feel the pain and I went all weekend with this pain. On monday morning while in the shower I started contracting more around my tummy. An hour later I was in active labor and 5 cm dialated. They had my baby out in 45 mins of arriving at the hospital.
Everyone is different.