Bianchi terms this fairly common condition the neurasthenia of pregnancy.
Vinay, Traité des Maladies de la Grossesse, 1894, pp. 51, 577; Mongeri, "Nervenkrankungen und Schwangerschaft." Allegemeine Zeitschrift für Psychiatrie, bd. LVIII, Heft 5. Haig remarks (Uric Acid, sixth edition, p. 151) that during normal pregnancy diseases with excess of uric acid in the blood (headaches, fits, mental depression, dyspepsia, asthma) are absent, and considers that the common idea that women do not easily take colds, fevers, etc., at this time is well founded.
Founding his remarks on certain anatomical changes and on a suggestion of Engel's, Donaldson observes: "It is impossible to escape the conclusion that in women natural education is complete only with maternity, which we know to effect some slight changes in the sympathetic system and possibly the spinal cord, and which may be fairly laid under suspicion of causing more structural modifications than are at present recognized." H. H. Donaldson, The Growth of the Brain, p. 352.
The state of menstruation is in many respects an approximation to that of pregnancy; see, e.g., Edgar's Practice of Obstetrics, plates 6 6 and 7, showing the resemblance of the menstrual changes in the breasts and the external sexual parts to the changes of pregnancy; cf. Havelock Ellis, Man and Woman, fourth edition, Chapter XI, "The Functional Periodicity of Woman."