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Mechanism of action of oxytocin

Gq receptors - increase contractility of uterine myometrium (increased amplitude/frequency)
Supplemented by prostaglandins - contraction (Via PLC) or relaxation (via cAMP)

Uterine priming

Estrogen sensitizes the myometrium to oxytocin - increases gap junctions and number of receptors

Two theories of initiation of birthing

Increased estrogen/progesterone ratio
Increased levels of CRH

Explain theory of increase estrogen/progesterone ratio

Explain theory of increased levels of CRH

Changes in uterine structure before birthing

Two distinct sections (upper muscular section - for propulsion; lower thinner section - allows for ripening of cervix and vagina)
Increased number of oxytocin receptors

Positive feedback loop that leads to birthing

First stage of labor

Begins when uterine contractions are of sufficient freq, intensity and duration
Latent phase: rate of cervical dilation is slow (up to 3cm)
Active phase: rate of dilation is about 1cm/hr

Second stage of labor

Begins when dilation reaches 10cm.
Fetal head begins descent (Friedman curve - fetal head in relation to ischial spine to assess progress of labor)

Third stage of labor

Interval between delivery of the fetus and delivery of the placenta
During this stage the placenta separates and is expelled
Typically takes 5-30 minutes
Causes bleeding - but limited by continued uterine contractions (vasoconstriction/vasospasm)

Physiology of dystocia

Abnormally slow labor
Can be cause by: uterus, size of the fetus, positioning of the fetus and excess sedation/anesthesia
Cephalicopelvic disproportion - head is too big to fit through birth canal

Breast milk is good because:

- It is suitable for digestion by immature GIT
- Provides optimal water/electrolytes
- Immunological protection
- Contains hormones (vitamin D, TRH, GnRH, etc)

Major hormones associated with lactation

- Estrogens, (progesterone, prolactin, hCS, cortisol, insulin, GH, and others)
- Epithelial tissues expand and become activated
- Estrogen and prolactin leads to deposition of fat tissues

Milk production inhibition in pregnancy

- High levels of estrogen (inhibits terminal differentiation) and progesterone (inhibits lactogenesis)
- After birth, both drop dramatically

Prolactin and lactation

- Suckling inhibits DA and stimulates prolactin releasing factor
- Therefore, suckling creates a prolactin spike
- Needs multiple w/in 24-hours to sustain milk production

Oxytocin and lactation

- Allows milk to be ejected from the breast (milk let-down reflex)
- Suckling leads to release (~1min delay)
- Oxytocin contracts myoepithelial cells leads to breast milk release

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