Final answer:
The most common fetal position at the onset of labor is the occiput anterior vertex, facing the maternal spinal cord, allowing for easier vaginal birth. Breech presentations are less common and usually result in Caesarean sections.
Step-by-step explanation:
The most common position of the fetus at the onset of labor is the vertex presentation, specifically the occiput anterior vertex position. In this position, the fetus faces the maternal spinal cord, and the smallest part of the head, which is the posterior aspect called the occiput, exits the birth canal first. This orientation is associated with the greatest ease of vaginal birth. Conversely, in fewer than 5 percent of births, the infant is in a breech presentation, which means buttocks down. There are different forms, such as the complete breech where both legs are crossed and oriented downward, or the frank breech where the legs are oriented upward. Today, most breech births are delivered by Caesarean section, unlike in the past where vaginal delivery was more common for breech presentations.
The stages of childbirth are crucial and knowledge of the typical fetal position is important for managing labor and delivery. The full-term fetus in the vertex position also plays a role in stimulating uterine contractions that lead to true labor.