The function of the human uterus is to support and protect a developing human embryo, as well as supporting other aspects of the female sexual response by directing blood flow. On a cycle that usually repeats approximately every 28 days, the uterus builds a lining to receive a fertilized egg.
The human uterus is a small but extremely flexible organ, able to accommodate human offspring through a tremendous change in size, from a single cell to a newborn baby, together with a placenta and an endometrial lining. This lining is a special tissue with a large blood supply meant to supply nutrients to a growing baby. If no fertilized egg implants into the endometrial lining, it is shed at the end of the 28-day cycle.
The uterus is made up of three layers of muscle that function despite stretching many times their normal extension. They are supported by a network of blood vessels and protein fibers. These walls grow thinner as the offspring develops, and only gain a small amount of mass. After birth, the uterus returns to its normal size in a few weeks, but it is never quite as small as before pregnancy. Generally, the uterus is permanently expanded by less than one-half inch in every direction.