The fourth day of menstruation is not pregnant, mainly considering two factors: First, in the menstrual period estrogen, progesterone levels are very low, ovarian function is also at a low level, there will be no egg development, only sperm without eggs is impossible to get pregnant. Secondly, during menstruation, there is vaginal bleeding, vaginal bleeding can flush sperm, so it is not conducive to sperm travel, sperm is difficult to enter the uterine cavity through the vagina. Even if there are sperm entering the uterine cavity, the peristaltic direction of the uterine myometrium at this stage is from top to bottom, which is not conducive to sperm traveling in the uterine cavity, and it is difficult for sperm to pass through the uterine cavity to the fallopian tubes. Therefore, combining the above two factors, intercourse on the fourth day of the menstrual period will not be pregnant.