With a 31-day menstrual cycle, there is also a risk of unwanted pregnancy if you have intercourse on the 11th day. When a woman has a regular menstrual cycle of 31 days, ovulation occurs on days 12-21 of the menstrual cycle, and the chances of pregnancy are higher when you have intercourse during this period. However, it is not true that other times are absolutely safe and there is no possibility of pregnancy. If you have intercourse on the 11th day of your period without using birth control, you are likely to get pregnant. Female ovulation is not necessarily fixed on a certain day, it is very easy to be affected by external environment, such as diet, environmental changes, stress and other factors, so that ovulation advance or push back, resulting in the safe period is not safe. Therefore, the safe period contraceptive method is not desirable, it is easy to contraceptive failure, resulting in unwanted pregnancy. If a woman has no plan to get pregnant, she can use effective ways of contraception such as intrauterine devices, oral contraceptive pills and wearing condoms. In order to avoid unwanted pregnancy, abortion to affect the health.