A summary of physiological changes in pregnancy

Pregnancy is a state of well-tolerated parasitosis. In order to adapt to such an abnormal demand, the maternal organism undergoes a seres of complex changes, in order to survive the anatomically ridiculous task of pushing a fully formed human being through an pelvic outlet clearly meant for something with a much smaller brain. These changes are described below in the form of a point-form list, all the better to slot into viva practice and SAQ revision.

Of the O&G-themed SAQs, apart from HELP and pre-eclampsia nothing else gets as much attention from the college, and so this topic is work spending some time on. Previous SAQs have consisted of the following:

Because these topics are fascinating, some additional attention has been expended on them in separate chapters, which can be safely omitted from the panicked last-minute reading of the time-poor candidate. As far as formal published stuff goes, one cannot go past the 2016 article by Soma-Pillay, "Physiological changes in pregnancy." Unless otherwise noted, this article was the main resource for this summary.

Now; those physiological changes, all on one page and in point-form as promised:

Airway changes

Bag-mask ventilation becomes more difficult:

  • The nasal mucosa is engorged, which means there is greater resistance to flow
  • The upper airway mucosa is oedematous
  • There has been weight gain

Laryngoscopy becomes more difficult:

  • Upper airway oedema
  • Breast enlargement
  • The Mallampatti grade changes during pregnancy, largely because of oedema of the pharynx, and due to weight gain. It gets even worse with labour.

Less time is available for intubation:

  • Decreased FRC, less time to intubate
  • Increased oxygen consumption, less time to intubate

Intubation is more risky

  • Increased risk of aspiration, decreased stomach emptying

Respiratory changes

  • The diaphragm is pushed up by 4cm
  • Tidal volume increases by ~ 30-50%
  • Respiratory rate increases to 15-17
  • Minute volume increases by 20-50%.
  • Chest wall compliance decreases 
  • Lung compliance remains the same 
  • FRC decreases during pregnancy, due to compression of the diaphragm by the gravid uterus.
  • pH increases to 7.40-7.47
  • PaCO2 decreases to 30 mmHg
  • PaO2 increases to 105 mmHg
  • HCO3- decreases to 20 mmol/L
  • Maternal 2,3-DPG increases 
  • p50 remains the same because of alkalosis 

Circulatory changes

  • Cardiac output increases (from 5L/min to 7L/min)
    • Stroke volume increases (from 65ml to 80-90ml)
    • Heart rate increases (from 75 to 85-90)
    • Systemic vascular resistance decreases (down by as much as 40%) - in fact, the vascular system becomes fairly refractory to the effects of vasoconstrictors such as angiotensin and vasopressin
    • The IVC is compressed by the gravid uterus in the supine position, decreasing the preload
  • Blood pressure decreases (and is lowest in the second trimester)
  • Pulmonary vascular resistance decreases
  • Pulmonary artery wedge pressure remains unchanged
  • Blood volume is increased by 50%
  • CVP remains unchanged
  • Colloid oncotic pressure decreases
  • Oxygen consumption increases by 20% during pregnancy

Electrolyte and endocrine changes

  • Vasopressin release increases;
  • Thus, there is water retention
  • A hypervolemic hypoosmolar state develops
  • In response to a decreased SVR, aldosterone release is increased. This is the major contributor to the 50% circulating volume expansion
  • There is a relative iodine deficiency (the foetus is stealing it all)
  • Cortisol secretion is increased, which has implications for all those people who still do random cortisol levels on their patients

Renal changes

  • Renal blood flow increases: the renal arteries are also affected by the fall in SVRI, and this is mediated by relaxin (which influences endothelial nitric oxide production).
  • GFR increases by as much as 85%
  • Urea and creatinine decrease because of this
  • Kidneys become enlarged; the renal pelvis dilates and there is a "physiological hydronephrosis" - more so on the right because the right ureter crosses iliac and ovarian vessels at an angle. This predisposes to pyelonephritis
  • Tubular resorption of urate and glucose decreases

Gastrointestinal and nutritional changes

  • Nausea and vomiting: in 50-90%. 
  • Oesophageal sphincter tone is decreased (aspiration is more likely)
  • There is increased intragastric pressure due to upward displacement
  • Gastric emptying is delayed, and is virtually non-existant during labour
  • Thiamine supplementation is important, because prolonged hyperemesis can result in vitamin deficiency.
  • Abdominal compartment pressure measurements are going to be wildly inaccurate.
  • There is insulin resistance, particularly later in pregnancy
  • Metabolic fuel use favours lipolysis, preserving the glucose and amino acids for use by the foetus.
  • Protein catabolism is decreased
  • There is a peak of calcium demand in the third trimester

Haematological changes

  • The overall trend is towards hypercoagulability. In the third trimester, coagulation activity is about double that of normal. The best source to read further is probably the 2003 article by Katarina Bremme
  • Platelet count decreases, particularly in late pregnancy
    • Normal pregnancy is associated with a degree of enhanced platelet destruction which is compensated for by increased production
    • The destruction takes place in the uteroplacental circulation
  • Some coagulation factor levels change substantially, and others do not. From Moore & Pereira (2013), as well as Katz & Beilin (2015):
    • Factors VII, VIII, IX, X, XII and von Willebrand factor increase significantly; Factor VII may increase as much as tenfold.
    • Factor XI decreases down to 60–70% of the non-pregnant value
    • Factors II and V do not change in pregnancy
    • Fibrinogen levels increase throughout pregnancy
    • Protein S levels decrease progressively during pregnancy
    • Protein C activity is unaffected by pregnancy
  • Plasma fibrinolytic activity decreases throughout pregnancy, but returns to normal within one hour of delivery. This is due to synthesis of plasminogen activator inhibitor-1 and -2 by the placenta

Tabulated changes to the respiratory and circulatory systems

The college love tables. If ever called upon to reproduce a table, these should come handy:

Respiratory Changes in Pregnancy
The diaphragm is pushed up by 4cm
Tidal volume increases by ~ 30-50%
Respiratory rate increases to 15-17
Minute volume increases by 20-50%.
Chest wall compliance decreases  
Lung compliance remains the same  
pH increases to 740-7.47
PaCO2 decreases to 30 mmHg
PaO2 increases to 105 mmHg
HCO3- decreases to 20 mmol/L
Maternal 2,3-DPG increases  
p50 remains the same because of alkalosis  
Circulatory Changes in Pregnancy
Heart rate Increased (from 75 to 85-90)
Stroke volume Increased (from 65ml to 80-90ml)
Cardiac output Increased (from 5L/min to 7L/min)
Blood pressure Decreased
Systemic vascular resistance Decreased
Pulmonary vascular resistance Decreased
Pulmonary artery wedge pressure Unchanged
Blood volume Increased by 50%
CVP Unchanged
Renal blood flow Increased by 30-80%
Resting oxygen consumption Increased by 20-30%
Colloid oncotic pressure Decreased


Munnur, Uma, Ben de Boisblanc, and Maya S. Suresh. "Airway problems in pregnancy." Critical care medicine 33.10 (2005): S259-S268.

Jeejeebhoy, Farida M., et al. "Management of cardiac arrest in pregnancy: a systematic review." Resuscitation 82.7 (2011): 801-809.

Kodali, Bhavani-Shankar, et al. "Airway changes during labor and delivery."Anesthesiology 108.3 (2008): 357-362.

Bremme, Katarina A. "Haemostatic changes in pregnancy." Best practice & research Clinical haematology 16.2 (2003): 153-168.

Moore, Lisa E., and Nigel Pereira. "Physiological changes of pregnancy." Maternal Critical Care: A Multidisciplinary Approach 107 (2013).

Katz, D., and Y. Beilin. "Disorders of coagulation in pregnancy." BJA: British Journal of Anaesthesia 115.suppl_2 (2015): ii75-ii88.