What is the general chance of pregnancy after stopping menstruation?

Patient's question:

What is the probability of getting pregnant from unprotected sex on the very first day of your period? Is it negligible? Thank you

Doctor's answer:

Hello: The chance of pregnancy is very small, and ovulation generally occurs around 14 days before the menstrual period.
It can also be calculated as follows:
　　First day of ovulation period = Shortest menstrual cycle days minus 18 days.
　　Last day of ovulation period = Longest menstrual cycle days minus 11 days.
Generally, doctors recommend that women with high-risk signs during prenatal examinations should maintain a balanced diet, avoid spicy and stimulating foods, as well as cold foods. Additionally, patients should create an environment that promotes emotional stability and stress relief. Wishing the patient a healthy delivery.