A hollow, triangular organ, flattened from front to back, the lower angle (or cervix) communicates through a narrow opening (the os uteri) with the VAGINA. The uterus or womb is where the fertilised ovum (egg) normally becomes embedded and in which the EMBRYO and FETUS develop. The normal uterus weighs 30–40 g; during pregnancy enormous growth occurs together with muscular thickening (see MUSCLE – Development of muscle). The cavity is lined by a thick, soft, mucous membrane, and the wall is chiefly composed of muscle fibres arranged in three layers. The outer surface, like that of other abdominal organs, is covered by a layer of PERITONEUM. The uterus has a copious supply of blood derived from the uterine and ovarian arteries. It has also many lymphatic vessels, and its nerves establish wide connections with other organs (see PAIN). The position of the uterus is in the centre of the PELVIS, where it is suspended by several ligaments between the URINARY BLADDER in front and the RECTUM behind. On each side of the uterus are the broad ligaments passing outwards to the sides of the pelvis, the utero-sacral ligament passing back to the sacral bone, the utero-vesical ligament passing forwards to the bladder, and the round ligament uniting the uterus to the front of the abdomen.