How to calculate menstrual cycle accurately?

Question
Hello doctor,
Last month, I had my period on the 18th. This month, I also got my period on the 18th, how many days is my menstrual cycle? Can you advise me on how to calculate my menstrual cycle accurately?
Le Thi Thao (1990)
Answer
Answered by Specialist II Tran Thi Mai Huong - Obstetrician and Gynecologist - Department of Obstetrics and Gynecology - Vinmec Hai Phong International General Hospital.
Hello,
With the question “How to calculate the exact menstrual cycle? ”, the doctor would like to answer as follows:
When a woman reaches puberty, menstruation is an important milestone that shows that a woman has begun to be fertile. During the first 2-3 years of puberty, ovarian function has developed but has not really been completed, the menstrual cycle in women has not yet entered a regular cycle. After that, the ovaries gradually complete and menstruation will follow certain rules to form the menstrual cycle.
Menstrual cycle is counted from the first day of menstruation until the first day of the next period. Some women have cycles lasting only 20 days, others up to 40 days or more, when cycles that repeat at a relatively steady rate are considered normal. So, your menstrual cycle is 30 days.
Menstrual cycle and ovulation day are closely related. To determine the exact day of ovulation, a woman's menstrual cycle must be regular and stable. The easiest way to estimate ovulation is by counting backwards. In theory, you'd first need to figure out exactly when your next period should start, based on the number of days that repeat cycles were previously tracked. After determining the date of your next period, you just need to count back 14 days, which is the day you will ovulate in this cycle. Understood as a simple rule, any woman, no matter how long the cycle lasts, 14 days after ovulating, will start the next menstrual period. From there, we apply the opposite calculation method.
For example, if you have a 28-day cycle, the day you ovulate is day 14 (28-14=14); If you have a 32-day cycle, your ovulation will be on the 18th day (32-14=18), if you have a 20-day cycle, you will ovulate on day 6 (20-14=6).
If you still have questions about your menstrual cycle, you can go to a hospital under Vinmec Health System for further examination and advice. Thank you for trusting and sending questions to Vinmec. Wishing you lots of health.
Best regards!