Being overweight does not cause sterility. However, sterility and weight gain can both be caused by a hormonal disorder such as polycystic ovarian syndrome (PCOS). Irregular periods indicate a possible hormonal disorder. You can visit a gynecologist for a hormonal analyses (FSH, LH, estrogen, progesterone, testosterone, DHEA, prolactin), colposcopy, Pap-smear, and vaginal ultrasound to help diagnose the condition.