A woman's menstrual cycle usually lasts from 28 to 30 days, but some women may have it longer or shorter. Understanding your menstrual cycle can help you better plan your diet and exercise plans, as well as better understand when safe and dangerous periods are.

First, let’s understand a woman’s menstrual cycle. Generally speaking, a woman's menstrual cycle is determined by the time when her period begins. If your menstrual cycle is 28 days, your next period will start on the 14th day of your current period. If your menstrual cycle is 30 days, your next period will start on the 16th day of your current period.

Next, let us understand the safe and dangerous periods for women. The safe period for women usually refers to the middle period of the menstrual cycle, that is, 7 days before and 8 days after the start of the next menstruation. This period is safe as women cannot become pregnant.

However, dangerous times are not always safe. The risk period refers to the time other than 7 days before and 8 days after the next menstrual period. During this period, women may become pregnant.

So understanding your menstrual cycle and following some basic rules can help you better understand when your periods are safe and dangerous. If you have a shorter menstrual cycle, you may need to wait until your cycle is longer to calculate the exact safe and dangerous periods. If you have longer menstrual cycles, you can try using some online tools or apps to calculate safe and dangerous periods.

Finally, let us remind you that even if you have calculated the safe and dangerous periods, you still need to take steps to protect yourself from the risks of pregnancy. If you have an accident during a safe or dangerous period, you may consider using emergency contraception, such as oral emergency contraceptive pills or condoms.

To sum up, understanding your menstrual cycle and following some basic rules can help you better understand when your safe and dangerous periods are. If you encounter any problems during your cycle, don't hesitate to ask for help.