36. Uterine Growth During Pregnancy and Parturation

  1. The nonpregnant uterus lies within the pelvis
  2. With pregnancy
    1. THE UTERUS grows in size, increases in weight, and its walls thin out
    2. DURING THE FIRST TRIMESTER, THE UTERUS rises out of the pelvic cavity and reaches the level of the umbilicus by week 20
      1. By weeks 28-30, the uterus reaches the epigastric region
      2. The mother's abdominal viscera are displaced, and the skin and muscles of the anterior abdominal wall are greatly stretched
    3. UTERINE SIZE INCREASE is due to hypertrophy of preexisting muscle fibers for the most part and partly to development of some new fibers
  3. Parturition or labor: the process of expelling the fetus, placenta, and fetal membranes. The factors causing parturition are unclear, but opinion favors the intermittent release of oxytocin from the maternal neurohypophysis as being important in determining the strength and duration of uterine contraction once labor is established and may well initiate labor as well. Steroid quantity produced by the fetal adrenal cortex may trigger some event in the placenta or myometrium of the mother that effects the onset of labor
    1. STAGES OF LABOR are usually described as being 3
      1. Stage I: when there is evidence of progressive dilatation of the cervix
        1. Occurs with the onset of regular uterine contractions that are less than 10 minutes apart and are painful
        2. Ends with complete cervix dilatation
        3. Average duration of labor is about 12 hours for the first pregnancy and about 7 hours for the multiparous mother
      2. Stage II begins when the cervix is fully dilated and ends with delivery
        1. Average duration for the prima gravida is 50 minutes; for the multipara is 20 minutes
      3. Stage III begins when the baby is born and ends when the placental membranes are delivered
        1. Average duration of this stage is less than 30 minutes
    2. AFTER DELIVERY OF THE BABY, THE UTERUS continues to contract, and a hematoma is formed behind the placenta which separates it from the decidua
    3. AFTER PLACENTAL DELIVERY, MYOMETRIAL CONTRACTIONS constrict the spiral arteries that supplied the intervillous space
      1. Contractions are tonic and prevent excessive bleeding from the placental site
      2. A relaxed uterus is the most common cause of postpartum hemorrhage

uterine growth during pregnancy  and parturation: image #1