Prevent pregnancy by calculating the day of ovulation

Question
Yes, doctor, I want to ask: if my wife and I have a period on the 28th or 30th, when is it safe to have sex to prevent pregnancy? I read the above, but it's so confusing.
Bui Duc Anh (1995)
Hello doctor. The doctor told me that if my wife has her period from the 3rd of every month to the 8th, what stage of sex is not pregnant. Without medication? I would like to thank you.
Anonymous question
Hello doctor, let me know that my menstrual cycle is 29 days. On May 15, I had my period until May 20, and it stopped and my wife and I went back on the 10th day after the period ended. So the doctor asked me if my menstrual cycle is like that, what day is ovulation and if my wife and I travel on the 10th day after the period is over, will I be pregnant? I hope the doctor can help me. Thank you.
Anonymous question
Answer
To prevent pregnancy by calculating the day of ovulation, you need to keep track of the days in each menstrual cycle for at least 6 months. Then apply the formula to calculate the day of ovulation to actively prevent pregnancy.
Subtract 18 from the date range of the shortest menstrual cycle. For example, if the shortest menstrual cycle is 26 days, then take 26 - 18 = 8. That means from the 8th day onwards the possibility of conception is higher. Similarly, subtract 11. For example, if the interval between the longest periods is 30 days, then take 30 - 11 = 19 days. So the dangerous time is calculated from the 8th to the 19th day of the menstrual cycle.