Uterine action during labor


 Uterine changes during pregnancy


Uterus is principally composed of involuntary muscle fibers with varying amounts of C.T. of w the most important is collagen.


Functionally and anatomically 3 regions are identified:


Body (corpus)is the main part of the uterus and contains the growing conceptus

                          It has 40% of the ms. Fibers in nonpregnant.

                          It provides the main expulsive force during labor


CervixConnects the body to vagina


Isthmusanatomically it is part of the cervix

                 Histologically it resembles the body of the uterus


                 *its upper margin is the anatomical internal os (evident macroscopically as a constriction between the uterine cavity and the cervical canal)

                  *Its lower margin is the histological internal os recognized microscopically as the fibromuscular boundary between cervix and corpus and also the junction between endometrial and cervical glands.


The lower part of the uterus developed late in pregnancy from the isthmus (1cm---10cm at term) by relaxation and stretching of the isthmus which becomes incorporated into the upper segment.


Uterus increase in weight 10-folds during pregnancy (50gm-----1kg)


Mostly because of hypertrophy (increase muscle length), but also due to hyperplasia (increase number of muscle fibers)


The uterus accommodates the growing fetus by:

1-Hypertrophy of muscle

2-Hyperplasia of muscle

3- increase compliance of c.t fibers



N/B compliance results from plasticity of the c.t fibers i.e. ability to stretch without change in pressure.


Factors that help the uterus to expand:

1-Unravelling of the collagen fiber bundle

2-softening of the interfibrillar ground substance permits sliding of fibers

3-degenerative changes may occur.



These changes depends on hormonal and mechanical factors




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