It is impossible for a fertilized egg to attach on the day of menstruation. Because the follicle begins to develop slowly on the 5th or 6th day of menstruation, growing at a rate of 2mm per day, and ovulating on the 16th day of menstruation, it is impossible for the egg to be fertilized on the day of menstruation. If you have sex on the day of ovulation or during ovulation, the sperm can enter the uterine cavity, pass through the uterine cavity to the fallopian tube, and combine with the egg waiting there to form a fertilized egg. The fertilized egg will reach the uterine cavity after about 10 days, and it will not immediately settle in the uterine cavity, and it will settle after 2-3 days to find a suitable location, which is the whole process from intercourse to fertilization, so it is impossible for the fertilized egg to settle on the day of menstruation.