A 31-year-old primigravid woman comes to the physician for a prenatal visit. She is known to be HIV positive. She also has asthma, for which she uses an inhaler. She had a diagnostic laparoscopy at age 20 for pelvic pain and has had no other surgeries. She has no known drug allergies. Extensive counseling is given to the patient regarding veical transmission of HIV to the fetus. It is recommended to her that she take antiretroviral therapy during the pregnancy to decrease the veical transmission rate. It is also recommended to her that she have a scheduled cesarean delivery. After consideration of these options, the patient chooses not to take the antiretrovirals and opts for a vaginal delivery. This of the following represents the approximate risk of veical transmission (from the mother to the fetus) for this patient?