Does menstrual blood flow back to the uterus when the cup is full?

It is impossible for blood to flow back into the uterus from the head of a pin-sized cervix.

Menstrual flow includes, among other things, the mucous membrane of the uterus, which makes it thick and uneven, thus, it is impossible for blood to flow back into the uterus from the head of a pin-sized cervix.

If the cup fills up during the night, it often begins to slide downwards out of the weight of the flow, giving more space for the blood to flow into the vagina. If there is a lot of flow, the cup may under pressure start to leak as gravity is doing its job. The uterus is actively pushing the flow out during menstruation.