The menstrual cycle is divided into two phases 1. Follicular phase and 2. Luteal phase. Fertility is at the tail end of the follicular phase and this is when pregnancy can happen. The luteal phase is when there is active bleeding and the ladies at this time is not fertile. Therefore to answer you question, NO it is unlikely that a pregnancy will occur just after monthly periods.
However it is important to note that not all ladies have a regular cycle and purely depending on the siting of menstrual blood as a sign of safe days is not always effective. A lady may be having inter-menstrual bleeding and thus be fertile.