Having intercourse on the last day of your period will not cause pregnancy because you are not ovulating at this time and the absence of ovulation will not cause pregnancy. Ovulation usually occurs about 14 days before menstruation, or between 25-30 days if the menstrual cycle is normal. During menstruation, the endometrium is thin because of the shedding of the endometrium. The endometrium needs to be thickened to about 0.7 cm before conception can take place. At this time, the endometrium may be between 0.4 and 0.5 cm, which may not be able to be used for implantation and thus may not cause pregnancy. If you are particularly unsure, you can also start counting from the time of intercourse and go to the pharmacy on the 12th day to buy an early pregnancy test, the results are also accurate. It is generally recommended that intercourse during menstruation is prohibited because the cervix is open during menstruation. If you have intercourse, it may cause pelvic inflammatory disease, tubal inflammatory disease and endometritis. In severe cases, abdominal pain, fever, and even dripping menstruation may occur.