There are two main hazards of sex during physiological period, the first point, retrograde infection; the second point, pelvic endometriosis. Physiological period cervical mouth is in an open state, and menstrual blood as a good culture medium for bacteria, at this time coitus is prone to retrograde infection, resulting in endometritis, adnexitis, and even pelvic inflammatory disease, the long-term complications of chronic pelvic pain, lower abdominal cramping and discomfort, and may even lead to ectopic pregnancy, infertility and so on. Menstrual blood reflux may occur during menstruation, leading to pelvic endometriosis. Pelvic endometriosis is a benign condition but has malignant symptoms that may lead to serious complications such as dysmenorrhea and infertility.