Final answer:
During the first stage of labor, the pain experienced is caused by myometrial hypoxia linked to uterine contractions that result from the stretching of myometrium and cervix by the full-term fetus.
Step-by-step explanation:
The primary cause of pain during the first stage of labor is attributed to the stretching of the myometrium and cervix by a full-term fetus. This stretching is a stimulant to uterine contractions, which initiate the regular contractions known as true labor. These contractions become more powerful and more frequent over time, leading to the dilation of the cervix, necessary for the baby and placenta to be expelled during birth. The diameter of the cervix will eventually expand to about 10 cm. Myometrial hypoxia, caused by these uterine contractions, is the primary reason for the pain felt during labor.
There are three stages to labor. The first involves cervical dilation, during which the cervix thins and widens to accommodate the passage of the baby. The second stage is the expulsion of the baby from the uterus, where strong contractions and maternal effort combine to deliver the baby. The final stage consists of delivering the placenta once the baby has been born. If labor stalls before the second stage, synthetic oxytocin known as Pitocin can be administered to help resume and maintain labor.