The most common causes are (in no particular order) infections, interstitial lung disease, and pulmonary oedema. Diagnosis
Type 1
Type 1 respiratory failure is defined as a low level of oxygen in the blood (hypoxemia) with either a standard (normocapnia) or low (hypocapnia) level of carbon dioxide (PaCO2) but not an increased level (hypercapnia). It is typically caused by a ventilation/perfusion (V/Q) mismatch; the volume of air flowing in and out of the lungs is not matched with the flow of blood to the lungs. The fundamental defect in type 1 respiratory failure is a failure of oxygenation characterized by:

This type of respiratory failure is caused by conditions that affect oxygenation, such as:

Low ambient oxygen (e.g. at high altitude)
Ventilation-perfusion mismatch (parts of the lung receive oxygen but not enough blood to absorb it, e.g. pulmonary embolism)
Alveolar hypoventilation (decreased minute volume due to reduced respiratory muscle activity, e.g. in acute neuromuscular disease); this form can also cause type 2 respiratory failure if severe. Diffusion problem (oxygen cannot enter the capillaries due to parenchymal disease, e.g. in pneumonia or ARDS)
Shunt (oxygenated blood mixes with non-oxygenated blood from the venous system, e.g. right to left shunt)

Type 2
Hypoxemia (PaO2 <8kPa or normal) with hypercapnia (PaCO2 >6.0kPa). The basic defect in type 2 respiratory failure is characterized by:

Type 2 respiratory failure is caused by inadequate alveolar ventilation; both oxygen and carbon dioxide are affected. Defined as the buildup of carbon dioxide levels (PaCO2) that has been generated by the body but cannot be eliminated.
According to Claude Levi-Strauss, the custom of couvade reinforces the institution of the family in some societies by "welding" together men and their wives and future children. See also
Couvade syndrome

References
External links
Chisholm, Hugh, ed. (1911). "Couvade" . Encyclopædia Britannica. Vol. 7 (11th ed.). Cambridge University Press. pp. 337–338. Couvade in Tribal Cultures