Chapter 36. Uterine Growth During Pregnancy and Parturation - Review of Medical Embryology Book

THE UTERUS grows in size, increases in weight, and its walls thin out
DURING THE FIRST TRIMESTER, THE UTERUS rises out of the pelvic cavity and reaches the level of the umbilicus by week 20

By weeks 28-30, the uterus reaches the epigastric region
The mother's abdominal viscera are displaced, and the skin and muscles of the anterior abdominal wall are greatly stretched

UTERINE SIZE INCREASE is due to hypertrophy of preexisting muscle fibers for the most part and partly to development of some new fibers

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

Occurs with the onset of regular uterine contractions that are less than 10 minutes apart and are painful
Ends with complete cervix dilatation
Average duration of labor is about 12 hours for the first pregnancy and about 7 hours for the multiparous mother

Average duration for the prima gravida is 50 minutes; for the multipara is 20 minutes

Stage III begins when the baby is born and ends when the placental membranes are delivered

AFTER DELIVERY OF THE BABY, THE UTERUS continues to contract, and a hematoma is formed behind the placenta which separates it from the decidua
AFTER PLACENTAL DELIVERY, MYOMETRIAL CONTRACTIONS constrict the spiral arteries that supplied the intervillous space