What is the pathophysiology of normal spontaneous vaginal delivery?

Normal spontaneous vaginal delivery is a physiological process involving complex interactions between maternal and fetal factors. The process can be divided into three stages:

1. Dilation and effacement of the cervix:

- Dilation: The cervix, which is normally closed during pregnancy, begins to dilate or open to allow the passage of the baby's head. This process is mediated by the release of hormones, including oxytocin, which stimulate contractions of the uterine muscles. The contractions put pressure on the cervix, causing it to thin out and gradually dilate.

- Effacement: As the cervix dilates, it also thins out or effaces. This occurs as the lower part of the uterus, known as the lower uterine segment, stretches and draws the cervix upwards. Effacement is expressed as a percentage, with 100% effacement indicating complete thinning and expansion of the cervix.

2. Descent and expulsion of the fetus:

- Once the cervix is fully dilated and effaced, the fetus begins to descend into the birth canal or vagina. This is aided by continued uterine contractions and the force exerted by the mother's abdominal muscles during pushing.

- As the fetus descends, the head engages with the pelvic bones and rotates to an optimal position for delivery. This usually involves the baby's head turning so that the face is towards the mother's back.

- The fetus then continues to descend and is eventually expelled from the vagina, completing the birth process.

3. Delivery of the placenta:

- After the baby is born, the placenta, which has provided nourishment and oxygen to the fetus during pregnancy, separates from the uterine wall.

- Mild uterine contractions resume, leading to the expulsion of the placenta through the vagina. This is known as the third stage of labor.

The duration of normal spontaneous vaginal delivery can vary widely among women, influenced by factors such as the mother's physical condition, the position and size of the fetus, and the strength and coordination of uterine contractions.