A 19-year-old G1P0 woman at 28 weeks gestation comes to labor and delivery because of the onset of contractions. The patient describes the contractions as progressively becoming more painful, each lasting 40 seconds and now occurring every five minutes. She reports good fetal movement and does not have any bleeding or leakage of fluid. On evaluation in triage, it is noted that she is having regular contractions, approximately every five minutes, has intact membranes, and her cervical exam is 3 cm dilated and 50 percent effaced. What is the most frequent cause of this condition