In many women, the opening of the vagina is partly or completely blocked by a thin tissue called the hymen. Some women are born without a hymen. The hymen usually breaks when a woman has sexual intercourse for the first time, that may cause or not some bleeding depending particularly on each woman. In some cultures, a broken hymen is taken as a sign that a girl has had sex. However, a broken hymen is not a clear sign of sexual intercourse because other activities such as exercise can also cause the hymen to break. In some women, the hymen may remain unbroken even after sex.