Passage/Pelvis:

This refers to the birth canal through which the baby must pass, including the mother's pelvis and soft tissues (cervix, vagina, and perineum). The size, shape, and flexibility of the pelvis, as well as the ability of the soft tissues to stretch, are crucial factors in determining whether a vaginal delivery will be successful.

Passenger:

The "passenger" refers to the baby. Factors like the baby's size, position, and presentation (whether the baby is head-down, breech, or in another position) play a significant role in the labor process. The alignment of the baby's head to the birth canal is also a key factor in the ease of delivery.

Power:

This refers to the strength of the uterine contractions during labor. The contractions must be strong and rhythmic enough to help push the baby down through the birth canal. "Power" also includes the mother's ability to bear down and push effectively during the second stage of labor.

Psyche:

The emotional and psychological state of the mother can significantly impact labor. Fear, anxiety, and stress can slow down or complicate labor, while relaxation, confidence, and a positive mindset can enhance the process. Support from birth partners and a calm environment can positively influence this aspect.