While using "safe period" as a contraception method, the unsafe period is:

Calendar method or the rhythm method depends upon the avoidance of sexual intercourse around ovulation. The Safe period is calculated from the first day of the menstrual period until the 10th day of the cycle and from the 18th to the 28th day for a woman with a regular 28 day cycle An alternative method is to calculate the Risk period, which is from 3 days before ovulation to 3 days after ovulation.