Final answer:
The most conclusive sign that uterine contractions are effective during the first stage of labor is the dilation of the cervix. True labor involves a cycle of uterine contractions leading to cervical dilation and effacement, and the process is regulated by oxytocin secretion.
Step-by-step explanation:
When assessing a woman in the first stage of labor, the most conclusive sign that uterine contractions are effective is the dilation of the cervix. True labor is characterized by a positive feedback loop where uterine contractions progressively stretch the cervix, leading to its dilation and effacement, or thinning.
As the cervix stretches, it further induces reflexive uterine contractions, boosting oxytocin secretion from the pituitary gland, which results in even more powerful contractions. The cervix thins and is dilated to about 10 cm during this first stage, which is necessary for the baby and placenta to be expelled during birth.