The term “vacation” refers to menstruation. The 30-day menstrual cycle, the 11th menstruation, the 20th coitus without measures generally will not be pregnant, but can not be absolutely excluded. The 30-day menstrual cycle, the 11th menstruation, if the next menstrual period is still 30 days apart, then the day of ovulation in the 27th, ovulation is the 22nd to the first of the next month, in the ovulation period of coitus will increase the chances of pregnancy, but in addition to other time is not absolutely safe, there is also the possibility of pregnancy. A woman’s menstrual cycle is affected by many factors, such as external stress and environmental influences, and the time of ovulation will also change, not necessarily on the projected date. If ovulation happens to be early this time, you may get pregnant if you have intercourse on the 20th. Therefore, the calculated safe period is not necessarily safe, or through effective contraceptive measures, such as condoms, birth control pills, etc. to prevent pregnancy.