Understanding Intense Menstrual Pain & Treatment by Dr OH So-young. Endometriosis When endometrial tissue grows outside the uterus, it may affect fertility, and its exact cause is unclear. Early detection is crucial. Polycystic Ovary Syndrome (PCOS): In their 20s and 30s, hormonal imbalances can lead to multiple ovarian follicles, reducing ovulation and disrupting the normal cycle. Treatment opt..