The hymen is a layer of tissue that partially conceals the vaginal orifice of some girls and women. It appears many if not most of us are under the impression the hymen is located within the vagina. It is, as the photographs and illustrations on this page reveal, part of the vulva, external genital organs. The hymen is also referred to as a girl's "cherry" or maidenhead.
During the early stages of fetal sexual development there is no external opening into the vagina. The layer of tissue that conceals the vagina at this time usually divides incompletely prior to birth, forming the hymen. The size and shape of the resulting opening(s) 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.*
The formation of this opening sometimes does not occur, resulting in an imperforate hymen. A doctor should examine a newborn girl's vulva to ensure a vaginal opening exists, as should her parents, because if menses is not permitted to flow freely from the body, pain and cramping may occur during menarche, a girl's first menstrual period(s).
The tissues of the vulva are generally thin and relatively delicate during childhood, as they haven't been influenced by the increased hormone levels that are present during puberty and the reproductive years; at birth the maternal hormones have had a visible influence on the vulva. Any activity that places tension on the vulvar tissues may stretch or tear the hymen. Some girls may tear their hymen while inserting a tampon for the first time.
It is commonly stated that girls and teens may tear or otherwise dilate their hymen while engaged in physical activities like sports, horseback riding, and masturbation, but there is no medical evidence to support this claim. This doesn't necessarily rule out that possibility, as very little research has been completed on the subject. The origin of these believes may have been an attempt at discouraging girls from engaging in activities seen as "masculine" in nature, coupled with the believe that a girl's most valued asset was her hymen/virginity.
Our survey indicates 49% of girls under the age of 15, and 73% under the age of 18, have inserted one or more fingers into their vagina while exploring their body, which may influence the appearance of their hymen. At the same age, 36% and 62% respectively, have stimulated their vagina with their fingers to produce sexual pleasure, which may also influence the appearance of their hymen.
"The elasticity of the hymen varies from woman to woman. Typically, though, with the introduction of a penis, fingers, or sexual devices, the opening stretches and becomes larger. Contrary to popular belief that the hymen can be ruptured by gymnastics, horseback riding, and other vigorous sports, no relation between sports and hymenal changes was found in a study of three hundred females. There is no medical definition of virginity based on the size of the opening of the hymen. The hymen can, as I said earlier, change in various ways, some occurring naturally during the first years of life." [Page 33]
"In 1994, a large study of three hundred young women showed that the median size of the opening in the hymen was slightly bigger in those who used tampons (1.5 cm or 5/8 inch)*** compared with those who used pads. But once again, it wasn't known what size opening each woman had before she ever used a tampon—so there's no way to know if the tampon enlarged the opening or not." [Page 90]
From: The V Book: A Doctor's Guide to Complete Vulvovaginal Health by Elizabeth G. Stewart, M.D. Copyright 2002.
Note: Conclusions presented in the first reference she cited have subsequently been refuted by other researchers, at least in regard to the potential consequences of tampon use. Reference  also presents evidence to support the belief that using tampons may alter the hymen. Injuries to the hymen could potentially be avoided if a young woman is familiar with her vulva, vagina, and proper tampon insertion techniques prior to using tampons.
Virginity is a spiritual attribute, not a physical one, the presence or absence of a hymen in no way indicates a girl's virginal state. No one can determine for certain, based on physical examination alone, whether a woman or teen has engaged in vaginal intercourse. Only around 50% (1 out of 2) experience bleeding the first time they participate in vaginal intercourse, meaning blood stained bed linen is not a reliable indicator of prior virginity.
The hymen of some women is injured and bleeds on more than one occasion; over 20% (1 out of 5) report they experienced hymeneal bleeding more than once. There are hymen that are elastic enough to permit a penis to enter without tearing, or tear only partially. A medical article indicates 52% of non-virgin adolescents in one study had "nondisrupted, intact hymens."
The hymen does not magically disappear when something is inserted into the vagina, it will only stretch or tear sufficiently to permit entry of whatever is being inserted. If for example a teen inserts two fingers into her vagina while masturbating, her hymen may still tear when she has vaginal intercourse for the first time, as the average penis is larger than her two fingers, and perhaps much less flexible and gentle.
A woman who has experienced vaginal intercourse may still have hymeneal tissue present, and this remaining tissue can be the cause of pain during intercourse. If a woman's current partner has a larger penis than her prior partners, or a couple tries a new technique or position during intercourse, her hymen may tear again, or for the first time.
When doctors examine preadolescent and adolescent girls for evidence of sexual abuse they look for injuries to the hymen; the hymen may still be intact except for a single tear. Remnants of the hymen, hymeneal caruncles (tags), are often present until a woman delivers a baby vaginally.
To learn what doctors have observed while examining patients please see these medical references.
A couple women have expressed concern about the shape of their vaginal opening, their introitus. Not being a medical doctor prevents me from determining if the appearance of their vulva is a reason for concern. The basis of their anxiety is likely the false believe that the area surrounding the vaginal opening should be flat and smooth.
This false expectation is addressed in the article about locating the vagina, which presents additional images of the introitus. While I cannot make a diagnosis concerning their anatomy, I can show how bumps and lumps in this area CAN BE perfectly normal. Only a doctor will be able to determine if your particular anatomy is a reason for concern.
The following photograph appears in a medical article intended to educate the medical community about the anatomy of the vagina. No indication of disease is made in the accompanying description of the photograph. The irregular shape of the tissues surrounding the vaginal opening, and the hymeneal remnants, could cause needless anxiety, if one presumes the area should appear otherwise.
The following photographs were submitted by visitors to the website, by those wanting to show normal variability, or in hopes of resolving concerns they had about their own anatomy. I cannot say whether these photographs show normal or abnormal anatomy. If after viewing these images you still have questions about your own anatomy, please consult your doctor. If your anatomy has always been as it is, that is less of a concern than if there has been unexplained recent changes.
* I haven't located a medical reference that substantiates this claim.
** Submitted to the website by a woman who was greatly concerned about the appearance of her hymen, prior to being reassured by her doctor.
*** For the study participants who had not engaged in vaginal intercourse, pad users had a median hymenal opening of 1.2 cm and the tampon users had a 1.5 cm opening . The difference between the two median measurements is 0.3 cm or 0.12 inches. To provide a frame of reference, the graphite material in a standard wooden pencil is equal to about half that difference, or 0.06 inches (1.6 mm).
1 Differences in hymenal morphology between adolescent girls with and without a history of consensual sexual intercourse. Arch Pediatr Adolesc Med. 2004;158:280-285. [PDF File]
2 Appearance of the Genitalia in Girls Selected for Nonabuse: Review of Hymenal Morphology and Nonspecific Findings. Journal of Pediatric and Adolescent Gynecology Volume 15, Issue 1, February 2002, Pages 27-35.
3 Appearance of the Hymen in Prepubertal Girls. PEDIATRICS Volume 89 No. 3 March 1992, pp. 387-394
4 Can tampon use cause hymen changes in girls who have not had sexual intercourse? A review of the literature. Forensic Sci Int. 1998 Jun 8;94(1-2):147-53.
5 Genital Anatomy in Pregnant Adolescents: "Normal" Does Not Mean "Nothing Happened" PEDIATRICS Volume 113 No. 1 January 2004, pp. e67-e69. [PDF File]
6 Hymenal findings in adolescent women: impact of tampon use and consensual sexual activity. J Pediatr. 1994 Jul;125(1):153-60.