A 26-year-old woman complains to her physician of discomfo during intercourse. Pelvic examination demonstrates a frothy, yellow-green vaginal discharge with a strong odor. Small, red, ulcerations of the vaginal wall are also seen. A wet mount preparation demonstrates motile, flagellated protozoa. If this woman had been pregnant and had not been treated, she and/or her baby would be at most significantly at increased risk of which of the following?