Is it harmful to have protected sex once during menstruation?
Generally speaking, "wearing a condom" refers to using a contraceptive condom. Having sexual intercourse with a condom during menstruation may also have adverse effects on a woman's body. The specific analysis is as follows:

During menstruation, a woman's cervix is slightly open and the endometrium is shedding, creating wounds. At this time, even with condom use, sexual activity may introduce pathogens such as bacteria and viruses into the vagina. These pathogens can enter the uterine cavity through the open cervix and cause pelvic inflammatory diseases such as endometritis and salpingitis. Moreover, a woman's immune system is relatively weakened during her period, decreasing the body's ability to resist foreign pathogens.
In addition, the stimulation during sexual activity, including genital friction and uterine contractions during orgasm, could interfere with the normal shedding and healing process of the endometrium. Uterine contractions might push the shed endometrial fragments into the fallopian tubes or even the pelvic cavity, causing endometriosis. At the same time, this stimulation may also lead to menstrual cycle disorders, such as increased or decreased menstrual flow, prolonged or shortened menstrual periods.
To protect a woman's physical and mental health, it is recommended to avoid sexual intercourse during menstruation. If discomfort symptoms have already appeared, seek timely medical attention and follow the doctor's advice for treatment.