Triggers of labor
- Decreased progresterone from placental \(\to \ \uparrow\)estrogen:progresterone ratio
- Oyxotocin release from maternal and foetal posterior pituitary
- Prostaglandins from foetal membranes
- Mechanicical uterine or cervical stretch
Peripartum uterine changes
- \(\uparrow\)myometrial excitability, \(\uparrow\)uterine oxytocin receptor expression
- Cervical softening, shortening, ripening
- Braxton-Hicks contractions \(\to\) eventual positive feedback from cervical stretch \(\to\) organized contractions \(\to\) labor proper
Stages of labor
- Cervical dilation - 6~12 hours, gradual dilation from uterine contraction until os ~10cm diameter. Visceral pain due to myometrial hypoxia during contractions
- Expulsion - Foetal head enters birth control \(\to\) birth of newborn; around 2 hours. Somatic pain due to birth canal tissue injury \(\to\) abdominal muscle contraction \(\to\) increased expulsive force
- Afterbirth - myometrium continues to contract, shears placenta from uterine wall; continued contraction \(\to\) reduces blood loss
CVS physiology
- Uterine contraction \(\to\) autotransfusion of maternal blood \(\to \ \uparrow\)preload
- Catecholamines \(\to\) increased TPR, HR
- Graded increase in CO, peaks at ~75% immediately postpartum
- SBP and DBP both increase
RESP physiology
- Pain, progesterone \(\to\) increased minute ventilation \(\to\) respiratory alkalosis, left-shifted maternal OHDC \(\to\) relative foetal hypoxia
GIT/METABOLIC physiology
- High metabolic rate, myometrial hypoxia \(\to\) lactic acidosis
- Decreased GI motility \(\to\) impaired drug absorption, aspiration risk