The exact cause of labor is unknown. Some of these theories that explain labor onset include:

1. Hormonal Changes: One theory suggests that labor begins when the balance of hormones in your body changes. The levels of progesterone, which has been maintaining the pregnancy, start to decrease while the levels of estrogen, which helps stimulate uterine contractions, start to increase.

2. Uterine Stretch: Another theory proposes that as the baby grows and the uterus stretches, this triggers the onset of labor. The stretching of the uterus is thought to stimulate signals which lead to contractions.

3. Oxytocin and Prostaglandins: These are substances produced by the body that can stimulate contractions. The theory suggests that the levels of these substances increase when it's time for labor to start.

4. Fetal Role: Some researchers believe that the fetus itself might trigger the start of labor. This could be due to proteins released by the lungs of a fully developed fetus.

5. Inflammatory Response: This theory suggests that labor is an inflammatory response, which is triggered by substances that stimulate inflammation being released in the uterus.

6. Corticotropin-Releasing Hormone (CRH): This hormone, produced by the placenta, is believed to play a key role in the timing of labor. The theory suggests that the level of CRH in the mother's blood increases towards the end of pregnancy and triggers labor.

Remember, these are just theories and the exact cause of labor is still not fully understood. It's likely that a combination of these factors, rather than just one, triggers the onset of labor.