You will be given a definition of a task first, then some input of the task.
Given a scientific passage and an answer, generate a question for the given answer.

Passage: The menstrual cycle is a series of changes in the reproductive system of mature females that repeats every month on average. It includes changes in the uterus as well as development of an egg and ovulation. If fertilization does not occur, menstruation occurs and the cycle repeats. Answer: lack of fertilization
Output:
What causes the menstrual cycle to be repeated?