The probability of pregnancy is very low if you have sex 1 day before your period. Because you are about to have your period and your eggs are dying at this stage, it is a relatively safe period and the probability of pregnancy is low if you have sex on this day. However, it is not recommended to use a safe period, or to calculate a safe period for contraception, as this method of contraception is not reliable. This is because ovulation can sometimes be affected by endocrine and environmental factors. If ovulation is delayed, the period of conception will be delayed and the probability of unwanted pregnancy will increase in this case.