Final answer:
The first stage of labor varies most in length. During this stage, the cervix thins and dilates to about 10 cm.
Step-by-step explanation:
The registered nurse tells the nursing student that the first stage of labor varies most in length. During this stage, the cervix thins and dilates to about 10 cm. This stage can take minutes, hours, or even days depending on various factors such as whether the person has given birth before.