It has been known since at least 1905 that pregnant female mammals gain bodyweight, additionally to the fetus and structures involved in reproduction, mostly early in pregnancy. Much of the substance gained in rats and possibly nearly all of it in humans is fat; it represents substantial storage of energy—typically 30,000 kcal (125.5 MJ) in the human. In a ‘state of nature’ such storage in advance of a time of increased demand for energy must be valuable; a similar mechanism is seen in birds before migration. Dewar showed that in mice the hormone progesterone, the signal for many of the changes in the mother in pregnancy, is also responsible for the weight gain. This action of progesterone has been extensively investigated in the rat (paper gives further references). Rats and mice are very suitable for such work, for they are available in uniform strains, are easy to handle, and their bodies can be analysed accurately. Cynics, however, have observed that a high proportion of endocrinology refers to the rat; it must not be assumed that the findings are necessarily true for other species.
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