During the early stages of fetal development there is no opening into the vagina. The layer of tissue that conceals the vagina at this time usually divides incompletely prior to birth. The size and shape of this opening or openings varies greatly from one girl to the next. There are girls who do not have a hymen at birth, as the tissue divides completely while they are still in the womb. Sometimes, this formation of an opening does not occur, resulting in an imperforated hymen.
The presence or absence of a hymen in no way indicates a girl's virginal state. No one can determine by physical examination alone whether a woman or teen has engaged in vaginal intercourse. Only about 50% of teens and women experience bleeding the first time they have intercourse, so blood stained bed sheets are not a reliable indicator of prior virginity. The hymen of some women tear on more than one occasion. There are even hymen that are elastic enough to permit a penis to enter without tearing, or tear only partially