Dangerous period of intercourse is not necessarily pregnant, only that the chances of pregnancy in this period is higher than other periods. The so-called dangerous period, is relative to the safe period, this period for women before and after ovulation, this stage if there is the behavior of coitus, and did not take effective contraceptive measures, the sperm and egg may meet after the formation of fertilized eggs, so it is easier to get pregnant, but can not ensure that must be pregnant, the reasons are as follows: 1, in the men and women are healthy under the premise of follicular discharge can be waiting for a day in the uterine cavity, sperm into the vagina can survive for 3 days, if there is no accurate calculation of the day of ovulation, in the male ejaculation has disappeared, then it will be more likely to get pregnant. The sperm can survive for 3 days after entering the vagina. If the day of ovulation is not calculated accurately and the follicle has disappeared when the man ejaculates, a fertilized egg cannot be formed; 2. Some women have problems such as tubal abnormality, polycystic ovary syndrome, scarred uterus and so on, which leads to abnormalities such as inability to ovulate on schedule and inability of fertilized egg to implant, and they cannot get pregnant; 3. Some men have sperm abnormality, which leads to the failure of fertilization or the formation of a normal Some men have sperm abnormalities that prevent successful fertilization or the formation of a normal fertilized egg, which still prevents the woman from having a successful pregnancy. If you are ready to get pregnant, you can have unprotected sex during the danger period. If you are ready to contracept, you must take protective measures during the danger period, such as taking contraceptive pills and wearing condoms.