When a woman is menstruating, pregnancy does not usually occur because the hormone levels secreted by the ovaries will continue to increase after pregnancy and will not cause the lining of the uterus to shrink. However, there are only a few special cases in which pregnancy may occur. 1, when a woman has a double uterus, if one uterus is pregnant and the other uterus is not pregnant, the other uterus that is not pregnant may have a menstrual period when it is time to menstruate. 2, it is also possible that a woman who is pregnant may have a miscarriage, which may result in bleeding in the vagina, and the woman may mistake it for menstruation. 3, the woman may be pregnant and the other uterus may not be pregnant. 4, the woman may not be pregnant. This can be mistaken for the onset of menstruation. Therefore, when a woman has menstruation and cannot rule out whether she is pregnant or not, she needs to go to the hospital for a blood test of chorionic gonadotropin levels or a gynecological ultrasound to make a clear diagnosis.