Complex sentence: In most women, various physical changes are brought about by natural fluctuations in hormone levels during the menstrual cycle, and by muscle contractions (menstrual cramping) involving the uterus that can precede or accompany menstruation.
Split into simpler sentences: In most women, various physical changes are brought about by fluctuations in hormone levels during the menstrual cycle. This includes muscle contractions of the uterus (menstrual cramping) that can precede or accompany menstruation.