The vagina is 8-10 cm in length, extending posterosuperior from the vestibule through the urogenital diaphragm to the uterus. The vagina forms a 90° angle with the uterus.
The vagina includes the following parts:
- vaginal orifice, incompletely covered by the hymen
- vault: upper end of the vagina
- fornices (anterior, posterior, lateral): recesses formed as the vagina surrounds the cervix
The anterior and posterior vaginal walls are usually closely apposed, diverging at the vaginal vault and fornices. The posterior fornix is covered by the peritoneum of the anterior part of the rectouterine pouch.
The vagina is supported by various structures:
- levator ani
- transverse cervical ligament
- pubocervical ligament
- uterosacral ligament
- perineal membrane and perineal body
After menopause, the vagina shortens in length and the fornices almost completely disappear.
- anteriorly: cervix, bladder, urethra
- posteriorly: pouch of Douglas, Denonvillier fascia, perineal body
- laterally: levator ani, pelvic fascia, ureters
- vaginal, uterine, internal pudendal and middle rectal arteries (branches of internal iliac arteries)
- vaginal venous plexus draining into internal iliac veins
- upper three-quarters: internal and external iliac, and sacral nodes
- lower part (below the hymen): superficial inguinal nodes
- upper vagina: includes both parasympathetic and sympathetic innervation
- sympathetic fibers from the hypogastric plexus supply blood vessels and smooth muscle of the vaginal wall
- afferent fibers run with sympathetic nerves
- lower 2-3 cm vagina: perineal and posterior labial branches of the pudendal nerve, anterior part of the vulva from the ilioinguinal nerve
Embryological derivation of the vagina is from two parts, which is important for understanding the origin of congenital anomalies:
- upper two-thirds of the vagina, cervix and uterus: all derived from the paired Mullerian / paramesonephric ducts.
- lower one-third of the vagina: derived from the bilateral sinovaginal bulbs which arise from the urogenital sinus
During transabdominal (TA) scanning the distended bladder, which acts as an acoustic window, does not affect vaginal position. The vagina can, therefore, be used as an effective landmark, even if the uterus does not occupy its familiar position in the pelvis. The vagina is best seen with a midsagittal TA approach, with a partially-filled bladder. The vagina is hypoechoic and the mucosa is echogenic. The echogenicity of the mucosa diminishes in menopause, with the loss of estrogen stimulation.