The first day of sex after menstruation, this situation is not pregnant. According to the normal situation, at this time, the follicle has not yet matured, there is no mature egg, it is impossible to form a fertilized egg, so you will not get pregnant, and the endometrium is relatively thin, even if a fertilized egg is formed, it is not suitable for the fertilized egg to be laid, so you will not get pregnant. Clinically, the period within 7 days after menstruation is called the safe period. After the safe period, if you do not want to have children, you need to take contraceptive measures during sex, such as oral emergency contraceptive pills or condoms, and ovulation is about 14 days before menstruation. If you have the need to have children, you can choose to have sex during this time, the chances of pregnancy are higher.