It is very unlikely to get pregnant on the third day of menstruation, but having sex during menstruation can easily lead to gynecological diseases, such as vaginitis, endometritis, and even pelvic inflammatory disease. Because a woman’s resistance is low when she has her period, the uterine opening is open, and the blood sinuses of the uterine cavity are also open, it is easy to bring bacteria to the uterine cavity and even retrograde infection to the pelvic cavity, causing endometritis or pelvic inflammatory disease. In addition, it may even bring active endometrium to the pelvic cavity, and planting to the pelvic cavity may cause endometriosis, which may affect future pregnancy or cause menstrual pain in the future. Therefore, it is best to wait until your period is completely clean before having intercourse. Although the probability of pregnancy is very small when you have your period, for safety reasons, it is best to wait until your period is completely clean before having intercourse.