The uterus is a hormone-tuned cradle for mammalian pregnancy
In most female mammals the uterus, or womb, shelters embryo and fetus until birth and secretes nourishing uterine milk from its lining. Humans have a fused simplex organ; some species keep a duplex pair. Clinical English prefers uterus; everyday speech still says womb.
Human anatomy places a pear-shaped organ behind the bladder and ahead of the sigmoid colon—about seven and a half centimetres long, four and a half broad, three thick, weighing near sixty grams. Fundus tops the tube openings; body fills the middle; isthmus narrows toward the cervix that gates the vagina. Fertilized eggs arrive via fallopian tubes at the uterotubal junctions.
The wall has three layers. Innermost, the endometrium splits into a lasting basal zone and a functional zone above it; that upper zone builds up and is shed in each menstrual or estrous cycle, while in pregnancy its glands and vessels multiply to form the decidua, and merging vascular spaces help build the placenta. The middle myometrium is mostly smooth muscle, and its innermost band, the junctional zone, thickens in adenomyosis. Outside lies the perimetrium, a serous covering of visceral peritoneum, wrapped in turn by the fibrous, fatty parametrium that ties the organ to neighbouring pelvic tissues.
Position matters as well. The pelvic diaphragm, perineal body and urogenital diaphragm give primary support, with the broad ligament and other peritoneal ligaments as backup. In most women the organ tips forward over the bladder, a pose called anteversion, and its body also bends forward on the cervix, called anteflexion. It is not fixed in place: a full bladder pushes it backward, a full rectum nudges it forward, and when both are full it rises. Even its interior is inhabited, by commensal and mutualistic microbes that make up a uterine microbiome.
Events inside are labelled in utero. As a sex organ that is also a temporary ecosystem for another genome, the uterus turns cyclic tissue remodeling into the possibility of birth.
Source: Uterus
