Final answer:
The first stage of childbirth is characterized by complete dilation of the cervix. It is the longest stage and can last several hours to a day or more. It allows the baby to move from the uterus into the birth canal.
Step-by-step explanation:
The first stage of childbirth is characterized by complete dilation of the cervix. During this stage, the cervix thins and can expand to a diameter of about 10 cm. It is an important milestone in the labor process as it allows the baby to move from the uterus into the birth canal. The first stage is typically the longest and can last several hours to a day or more.