During pregnancy, P4 domination has been exerting a quiescent effect on the mymetrium

Note the rise in fetal cortisol precedes all the hormonal changes occuring during parturition

Destruction of the fetal pituitary or adrenalectomy prolongs gestation

Timing of parturition is linked to genetic contraol of the fetal adrenal maturation

Initiation of Parturition

(Stress)

Fetus is proposed to perceive a stress

Perception of this stress activated the hypothalamic-pituitary-adrenal cortex axis to cause the release of fetal cortisol

Stressor is proposed to be a physical or mechanical factor

Ex: attainment of a critical fetal size & weight

Secretion of fetal cortisol dramatically changes placental endocrine function

Coritsol

(mechanism of hormone action)

Initiation of Parturition

(hormone changes)

• Decreasing P4 removes the block on myometriam contractility
• E2 production stimulates PGF-2alpha synthesis
• Both E2 and PGF-2alpha exert myokinetic effects on the myometrium and stimulate contractility
• PGF-2alpha stimulates relaxin release
• E2 upregulates relaxin and oxytocin receptors
• Enhanced contractility in the uterus causes the fetus to press against the cervix stimulating oxytocin release via neuroendocrine reflex arc

Initation of Parturition

(hormone flow chart)

Cholesterol-->

Pregnelone-->

Progesterone--(17alpha-hydroxylase)-->

17alpha-Hydroxyprogesterone--(17-20 Desmolase)-->

Androstendione-->

Testosterone--(Aromatase)-->

Estradiol

Parturition Cascade

Fetal Stress-->

Fetal Hypothalamus(perceives stress)

--(corticotropin releasing factor)-->

Fetal Puitary Gland

--(corticotropin)-->

Fetal Adrenal Cortex

--(cortisol)-->

Placenta (Target Tissue)

--> Decreased Progesterone

--> Increased Estradiol

--> Increased PGF-2alpha

--> Increased Relaxin

Parturition Cascade

(decreased progesterone....)

Parturition Cascade

(Increased Estradiol....)

• Increased secretion for lubication-->

Fetus & Fetal Membrane explusion

• Increased Oxytocin-->

Fetus and Fetal Membrane explusion

• Increased myometrial contractions-->

increased cervical stimulation-->

increased oxytocin-->

fetus and fetal membrane explusion

• increased PGF-2alpha

• increased relaxin

Parturition Cascade

(Increased PGF-2alpha...)

• Increased myometrial contractions-->

increased cervical stimulation-->

increased Oxytocin-->

Fetus and fetal membrane explusion

• Luteolysis

• Increased Relaxin

Parturition Cascade

(Increased Relaxin)

• Cervical dilation & relaxation of pelvic ligaments-->

fetus and fetal membrane explusion

Dilation of cervix

Maternal restlessness

elevated pulse and respiration rate

changes in fetal position and posture

Maternal recumbency and straining

Rupture of allantochorion and escape of fluid from vulva

appearance of amnion at vulva

rupture of amnion and delivery of fetus

maternal straining ceases

loosening of chorionic villi from maternal crypts

inversion of chorioallantois

straining and expulsion of fetal membranes

Cortisol stimulates surfacant production in the fetal lungs

Vascular changes

surfacatn decreases alveolar surface tension allowing lung expansion

preparation for breathing in extra-uterine life

• During fetal life, oxygenated blood is delivered to the fetus by the umbilical vein; the fetal lungs are bypassed
• closure of the foramen ovale- higher pressures in the left atria press a flap of tissue against the right atria closing the foramen ovale; occurs at birth
• construction on the ductus arteriosus and the ductus venosus- complete closure of both requires some months

Period of uterine involution and endometrial repair

Principal forces driving involution are myometrial contractions

Lochia

Due to sustained release of PGF-2alpha for uterine postpartum

augmented by oxytocin release during episodes of suckling