1.  The action of giving birth; childbirth.
Parturition
 The action of giving birth; childbirth.

2. Hormones and Chemicals Involved
Hormone/Chemical
Source
Function
Estrogen
Ovary/Placenta
Stimulates the myometrial cells of the uterus to form abundant oxytocin receptors, promotes formation of gap junctions between the uterine smooth muscle cells, and antagonizes progesterone’s quieting influence on uterine muscle.
Progesterone
Promotes growth/maintenance of endometrium and relaxes smooth muscle, which inhibits uterine contraction during pregnancy.
Corticotropin-Releasing Hormone (CRH)
Hypothalamus
Stimulation of the pituitary synthesis of ACTH; in the placenta, CRH is a marker that determines the length of gestation and the timing of delivery.

3. Hormones and Chemicals Involved (continued)
Hormone/Chemical
Source
Function
Adrenocorticotropic Hormone (ACTH)
Anterior Pituitary
Stimulates adrenal cortex to make and secrete glucocorticoids and androgens.
Cortisol (glucocorticoid)
Adrenal Cortex
Assist body to resist long-term stressors.
Oxytocin
Hypothalamus; stored in posterior pituitary
Stimulates uterine contractions during labor.
Prostaglandins
Most tissue cells
A lipid-based chemical messenger that acts locally as a paracrine to trigger the rhythmic expulsive contractions of true labor; play a major role in the thinning and softening of the cervix just before and during labor.

4. Initiation of Parturition
Fetal stress stimulates CRH, which in turn stimulates ACTH by the anterior pituitary and then stimulates cortisol release by the adrenal glands.
Cortisol release affects the placenta in three ways:
Involved in the maturation of the fetal lung.
Increases estrogen production.
Increases placenta CRH production, which causes dilation of the uterine vessels, stimulation of smooth muscle contractions, and stimulation of prostaglandins production.
Note: While CRH stimulates fetal ACTH release, the mother’s ACTH stimulates the adrenal that makes cortisol, which decreases the production of CRH, resulting in a decrease in ACTH secretion.
Positive and negative feedback may be controlled by different signal transduction mechanisms.

5. Initiation of Parturition (continued)
As the baby is pushing out, the uterine and cervix begin to stretch, which stimulates sensory nerve fibers around the area.
Sensory nerve fibers stimulate oxytocin production, causing uterine contractions and stimulating the uterus to produce more prostaglandins.
Cycle continues until baby is born—positive feedback mechanism.

6. Stages of Labor Dilation Stage:
As labor starts, weak but regular contractions begin in the upper part of the uterus and move toward the vagina.
As labor progresses, the contractions become more vigorous and rapid.
Infant’s head is forced against cervix, causing the amniotic sac to rupture.
Engagement occurs when the infant’s head enters the true pelvis and through the birth canal where the baby’s head rotates to navigate the narrow dimensions of the pelvic outlet.

7. Stages of Labor (continued)
Expulsion Stage:
Crowning occurs when the largest dimension of the baby’s head distends the vulva.
The baby’s neck extends as the head exits from the perineum.
After the head has been delivered, the rest of the baby’s body is delivered much more easily.

8. Stages of Labor (continued)
Placental Stage (afterbirth):
Delivery of the placenta and its attached fetal membranes.
The strong uterine contractions that continue after birth compress uterine blood vessels, limit bleeding, and shear the placenta off the uterine wall.

9. Miscellaneous
False labor (Braxton Hicks contractions): Irregular uterine contractions due to the rise in estrogen, which limits progesterone’s influence on uterine muscle.
Breech presentation: buttock-first presentation of the baby during labor, which usually requires C- section.