A hymen is a thin membrane that stretches across the vagina. It comes in all shapes and there is no one kind of hymen. Not everyone has a hymen and only a few people have a hymen at birth. The hymen generally breaks when there is some pressure received in the vagina, which could be as a result of some physical activity like running, swimming, cycling among other. When the hymen breaks, a small amount of blood is released from the vagina which is common. People believe that the hymen can only break when you have sex butq this is not true.