The inner one-third of the myometrium (termed the junctional or sub-endometrial layer) appears to be derived from the Müllerian duct, while the outer, more predominant layer of the myometrium appears to originate from non-Müllerian tissue and is the major contractile tissue during parturition and abortion.

[1] The myometrium stretches (the smooth muscle cells expand in both size and number[3]) during pregnancy to allow for the uterus to become several times its non-gravid size, and contracts in a coordinated fashion, via a positive feedback effect on the "Ferguson reflex", during the process of labor.

A positive benefit to early breastfeeding is a stimulation of this reflex to reduce further blood loss and facilitate a swift return to prepregnancy uterine and abdominal muscle tone.

[1] As noted for the macrostructure of uterine smooth muscle, the junctional layer appears to be capable of both peristaltic and anti-peristaltic activity.

Removal of Ca2+ after contraction induces relaxation of the smooth muscle, and restores the molecular structure of the sarcoplasmic reticulum for the next contractile stimulus.