24B17: Exam Report

Describe the physiological changes that occur in the pregnant person during parturition (labour).

(Changes that occur within the foetus are not required).

39% of candidates passed this question.

This question required consideration of the alterations of the uterus, and the hormonal changes during labour, as well as discussion of the effects of this on the other body systems.

Cardiovascular and respiratory changes held most of the mark weighting however gastrointestinal, metabolic and neuronal changes were also required.

A good answer was structured using the 3 stages of labour to explain the hormonal changes that initiate and maintain labour (oxytocin and positive feedback the primary but not the only factor described), changes to the cervix, uterine muscle and active muscular effort, delivery of neonate and placenta and uterine mechanisms to limit blood loss.

Using this structure a description of cardiovascular and respiratory changes should follow with some detail about pain / endorphins, abdominal pressure / LOS sphincter and metabolic changes (lactate / glucose utilisation) also expected.

The question was specific to parturition.

Physiological changes throughout pregnancy were not required.

V1i / 24B17: Describe the physiological changes that occur in the pregnant person during parturition (labour)

Definition

Parturition is a physiologic process involving a sequential, integrated set of changes within the myometrium, decidua, and cervix that occur gradually over a period of days to weeks, culminating in rapid changes over hours that end with expulsion of the products of conception (fetus and placenta).

You can use either of the two different stuctures

A. Anatomical / Uterine changes

  • Activation of amnion and decidual membranes, and
  • Conversion of uterine smooth muscle from being quiescent to contractile
  • Myometrium
    • Expresses more oxytocin receptors
    • Increases its sensitivity

Phase 1: Myometrial activation (priming)

  • Functional withdrawal of progesterone activity at the level of the uterus
    • Facilitates coordinated contractility
  • Cervical softening and ripening- rearrangement of the extracellular matrix of the cervix 

Phase 2: Stimulation

  • Oxytocin, PG E2 and F2 alpha
    • Facilitate contractions
  • Stretching of the myometrium and cervix by a full-term fetus
    • Stimulates uterine contractions

Phase 3: Involution

  • Days and weeks following delivery- usually complete by the sixth week postpartum
  • Mediated primarily by oxytocin
  • Returns to its normal prepregnant state, both structurally and functionally
  • Mechanisms- apoptosis and autophagy of myometrium and connective tissue, reduced uterine blood flow, endometrial vascularity, endometrial regeneration, and a resumption of ovarian function

Stucture B - Hormonal changes

Corticotropin-releasing hormone

  • Produced by placenta and fetus
    • Fetal cortisol rises
    • Boosts placental estrogen>overpowers effects of progesterone
  • Decrease in progesterone and corresponding increase in estrogen
  • Increases the actions of oxytocin and prostaglandins

Estrogen

Progesterone

  • “Functional progesterone withdrawal”
    • Transforms quiescent uterus to contractile state

Oxytocin

  • Facilitates the coordinated muscle contraction necessary for expulsion of the fetus and placental membranes
  • Positive feedback loop
    • Stimulate the secretion of prostaglandins from fetal membranes

Prostaglandins

  • Enhance uterine contractile strength

Relaxin

  • Increases the elasticity of the symphysis pubis joint and pelvic ligaments, making room for the growing fetus and allowing expansion of the pelvic outlet for childbirth
  • Also helps dilate the cervix during labor

Changes in the Fetus

(College did’nt want it specifically, but who knows this may be asked next!):
  • Stimulation of the hypothalamic-pituitary-adrenal axis of the fetus
    • Increased ACTH and adrenal cortisol
    • Initiation of parturition
  • Fetal pituitary also secretes oxytocin

Effects / changes in other systems:

CVS

  • Progesterone
    • Smooth muscle relaxation
    • Reduces vascular resistance
    • Decrease in arterial blood pressure

  • Uterine contraction squeezes blood to maternal circulation (auto transfusion), cardiac output increases (immediately after delivery up by about 60 – 80%) and blood pressure increases (both systolic and diastolic)

Respiratory

  • Progesterone
    • Increase the responsiveness of the respiratory system to CO2 or to be a primary stimulant
    • Increase in minute ventilation >increase in PaO2 levels and a decrease in PaCO2

Metabolism

  • Human placental lactogen (hPL)> induces lipolysis
  • Total serum cholesterol and triglyceride levels increase
  • Increased caloric and nutritional requirements
  • High metabolic state> high lactate levels

Hematology

  • Increase in blood cell volume
    • Increase in the risk of thromboembolic events starts in the first trimester and continues at least 12 weeks postpartum.

GI

  • Progesterone
    • Smooth muscle relaxation prolonged gastric emptying time
    • Decreased gastroesophageal sphincter tone and upwards displacement of the stomach by the gravid utterus
    • Reflux
  • Decreased motility in the large bowel
    • Increased water absorption and constipation

Endorphins

  • Increased in levels to combat the increasing pain during labour caused by myometrial hypoxia from the contractions

Stages of labour

Cervical Dilation

  • Must dilate fully to 10 cm in diameter—wide enough to deliver the newborn’s head. 
  • The longest stage of labor ~ 6–12 hours.
  • Positive feedback loop
    • Uterine contractions stretch the cervix, cervical dilation, effacement  / hinning
    • Iinduces reflexive uterine contractions
    • Dilate and efface the cervix further
  • Cervical dilation boosts oxytocin secretion from the pituitary
    • Triggers more powerful uterine contractions
  • Rupture of amniotic membranes before the onset of labor at the end of the dilation stage in response to excessive pressure from the fetal head entering the birth canal in about 12 percent

Expulsion Stage

  • Begins when the fetal head enters the birth canal and ends with birth of the newborn~ up to 2 hours.

Afterbirth

  • Delivery of the placenta and associated membranes
  • Myometrium continues to contract>shears the placenta from the uterine wall
    • Delivered
  • Continued uterine contractions then reduce blood loss from the site of the placenta

Postpartum period

  • Marked by delivery of the placenta up to ~6 weeks immediately following childbirth maternal body returns to a non-pregnant state

Author: Nazma Navilehal Rajasab