Final answer:
Vaginal exams to assess dilation and effacement of the cervix are typically begun when labor is imminent or based on medical indications, often around 36 weeks or later. The cervix must dilate to 10 cm for vaginal birth.
Step-by-step explanation:
When assessing dilation/effacement of the cervix via vaginal exams in preparation for childbirth, these are typically not performed routinely at any set time during pregnancy but may be considered when labor is suspected or for specific clinical indications. Regular vaginal examinations to assess cervical changes are not recommended until labor appears imminent or for medical reasons necessitated by the healthcare provider. Most commonly, this occurs around 36 weeks or later, but could occur earlier if there are signs of preterm labor or other concerns. However, this question seems to seek a standard answer without additional context, in which the option closest to the common practice would be 'c) 36 weeks' when such exams might begin in anticipation of labor. It is important to note that these examinations are part of the first stage of labor as the cervix must dilate fully to 10 cm to allow for vaginal birth.