The main reasons for this are as follows: 1. The condom is not used correctly, for example, the condom is worn only when the ejaculation is about to take place, in which case the prostatic fluid is secreted before the ejaculation, and the prostatic fluid contains sperm, which will cause pregnancy if left in the vagina. 2. The effect of oral contraceptives is not 100%, so even if you take the pill, clinically it may not prevent ovulation or changes in the uterine lining because of individual differences, which can still lead to pregnancy. Therefore, even if you wear a condom and take the pill at the same time, there is still a possibility of pregnancy, but the chance is relatively small.