There is a good article by Hegewald & Crapo which summarises the respiratory physiology of pregnancy in clear and concise terms. On the basis of this article, I have tried to compile a list of the relevant changes:

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  

All of this can probably be demonstrated in a big confusing diagram.

changes in respiratory volumes during pregnancy

Diaphram is pushed up by 4 cm

The diaphram is pushed up by 4cm on average.

Weirdly, not all of this is due to to the effect of the gravid uterus pushing up the abdominal contents.

Apparently the hormone-induced laxity of rib ligaments begins well before the uterus becomes large enough to shove any abdominal contents into the chest cavity. This, however, does not decrease the mechanical effectiveness of the diaphram, and if anything it works more efficiently during pregnancy, with an increase in diaphragmatic excusion by about 2cm.

Tidal volume increases by ~ 30-50%

The increased diaphragmatic excursion changes the tidal volume from (lets say) 500ml in the first trimester to over 700ml in the third trimester.

Respiratory rate increases from 15 to 17 on average; thus, minute volume increases by 20-50%.

This seems to be driven by progesterone, as will be discussed later.

Chest wall compliance decreases (but lung compliance remains the same)

As one might imagine, the workload of the ventilator turbine is somewhat increased by the increased weight of the chest wall, owing particularly to the large turgid breasts heaving around upon it. The changes in ribcage configuration and the upward incursion of the diaphragm also play a role, as the normal "bucket handle" mechanics become disturbed. Lastly, the gravid uterus provides a counterpressure to inspiration, in a manner similar to the distended abdomen of an abdominal compartment syndrome.


Oh's Intensive Care manual:

Chapter 64   (pp. 684) General  obstetric  emergencies by Winnie  TP  Wan  and  Tony  Gin

Chapter 65   (pp. 692) Severe  pre-existing  disease  in  pregnancy by Jeremy  P  Campbell  and  Steve  M  Yentis

Yeomans, Edward R., and Larry C. Gilstrap III. "Physiologic changes in pregnancy and their impact on critical care." Critical care medicine 33.10 (2005): S256-S258.

Hegewald, Matthew J., and Robert O. Crapo. "Respiratory physiology in pregnancy." Clinics in chest medicine 32.1 (2011): 1-13.

Zwillich, C. W., et al. "Effects of progesterone on chemosensitivity in normal men." The Journal of laboratory and clinical medicine 92.2 (1978): 262-269.

Moore, LORNA GRINDLAY, et al. "Maternal hyperventilation helps preserve arterial oxygenation during high-altitude pregnancy." Journal of Applied Physiology 52.3 (1982): 690-694.

Levinson, Gershon, Sol M. Shnider, and John L. Steffenson. "Effects of maternal hyperventilation on uterine blood flow and fetal oxygenation and acid-base status." Anesthesiology 40.4 (1974): 340-347.

Huch, Renate. "Maternal hyperventilation and the fetus." (1986). J.Perinat.Med 14 (1986) 3

Fadel, Hossam E., et al. "Normal pregnancy: a model of sustained respiratory alkalosis." Journal of Perinatal Medicine-Official Journal of the WAPM 7.4 (1979): 195-201.

Jensen, Dennis, et al. "Physiological mechanisms of hyperventilation during human pregnancy." Respiratory physiology & neurobiology 161.1 (2008): 76-86.

AWE, ROBERT J., et al. "Arterial oxygenation and alveolar-arterial gradients in term pregnancy." Obstetrics & Gynecology 53.2 (1979): 182-186.

Milne, J. A. "The respiratory response to pregnancy." Postgraduate medical journal 55.643 (1979): 318-324.

Madsen, Hans, and Jøm Ditzel. "Red cell 2, 3-diphosphoglycerate and hemoglobin-oxygen affinity during normal pregnancy." Acta obstetricia et gynecologica Scandinavica 63.5 (1984): 399-402.