How to calculate the next ovulation day when menstruation is delayed

Patient's question:

My wife is 29 years old this year. Her menstrual cycles were all 26 days before, but this time it's 31 days. Is it normal? What could be the cause? She took an emergency contraceptive pill in early July. Can she get pregnant next month? How to calculate her next ovulation day

Doctor's answer:

Hello: Generally, a delay or advancement of one week in the menstrual cycle is considered normal. Taking birth control pills may cause menstrual disorders, but based on your medication timeline, it should not have a significant impact, and pregnancy can be considered.
Calculation of ovulation period: It is not absolute and varies from person to person. Under normal circumstances, it is indicated by a rise in body temperature. Nowadays, many people plan to have children on purpose and are very concerned about how to calculate the ovulation period, how to know which day is the ovulation day using the basal body temperature method, and how to correctly perform this basal body temperature method.
For women with a regular menstrual cycle, the day of ovulation is actually relatively fixed, usually around 14 days before the next menstrual period. For example, if this month's period is on March 5th, the next period might be on April 5th, and the ovulation day could be around March 20th. For women with irregular menstrual cycles, it may be more difficult to determine the ovulation date. The simplest method is to measure the basal body temperature to identify the ovulation date. The correct method for measuring basal body temperature is to wake up in the morning without any physical activity, start taking the temperature in bed, and use an oral thermometer.
After taking the temperature, record the daily readings on a temperature chart. Continue monitoring until the temperature rises by 0.5 degrees compared to the previous day and remains elevated for three to five days. The day when the temperature rises may be the ovulation day. If you want to conceive during the ovulation period, the best time to have intercourse is one to two days after the temperature rises, as this has the highest conception rate.