GESTATION 



999 



eifect of litter size on length of gestation. 

 The smaller litter size gave a higher inci- 

 (l(>nce of prolonged gestation. 



Studies on oxygen consumption in the 

 guinea i)ig revealed a slight but significant 

 rise of 8 per cent at the end of gestation 

 (Hoar and Young 1957). The increase in 

 oxygen consumption is consistent but slight 

 for the first 60 days of pregnancy after 

 which the significant increase occurs (Fig. 

 16.32). The rise continued until 5 days 

 postpartum and then fell rapidly. In a sec- 

 ond set of experiments oxygen consumption 

 was measured in control, thyroidectomized, 

 and thyroxine-injected, pregnant guinea 

 pigs. Measurements were taken at the 

 time of mating and at parturition. In all 

 three instances, an increase in the oxygen 

 consumption was noted at parturition as 

 compared with the values at the time of 



TABLE 16.9 



Ejfcct of thyroid-deficiency and litter size on length 



of gestation in mice 



(From H. M. Bruce and H. A. Sloviter, J. 

 Endocrinol., 15, 72, 1957.) 



mating (Fig. 16.33). Again the control 

 guinea pigs showed a 7.9 per cent gain in 

 oxygen consumption by the end of preg- 

 nancy, but both the thyroidectomized preg- 

 nant guinea pigs and the thyroxinc-treated 

 guinea pigs also showed an increase in oxy- 

 gen consumption of 11.9 and 16.2 per cent, 

 respectively. The increase in oxygen con- 

 sumption was not paralleled by increases in 

 heart rate; actually the heart rate decreased 

 in several instances. In addition, neither 

 the weight of the thyroid gland nor the 

 histology of the gland was changed during 

 pregnancy. It is obvious then that an ex- 

 planation for the rise in oxygen consump- 

 tion during pregnancy may not involve the 

 thyroid gland. On the basis of changes in 

 its appearance. Hoar and Young (1957) 

 suggested the possibility that the adrenal 

 cortex is involved and that the increased 

 oxygen consumption is due to an increased 

 release of adrenal corticoids. More evidence 

 is needed before this suggestion can be fully 

 accepted. 



Further work from the same laboratory 

 has led to the concept that one locus of 

 action of thyroxine during pregnancy is at 

 parturition (Hoar, Goy and Young, 1957). 

 These investigators used an inbred strain 

 of guinea pigs that is characteristically hy- 

 pothyroid and a genetically heterogeneous 

 stock in which the level of thyroid activity 

 is presumed to be higher. It had been pre- 

 viously shown that pregnancy wastage was 

 high in the hypothyroid guinea pigs. Treat- 

 ment with thyroxine reduced the percentage 

 of stillborn from 40 to 13.6 in the hypothy- 



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Fig. 16.32. Oxvgen consumption in the guinea pig during gestation. (From K. M. Hoar 

 and W. C. Young, Am. J. Physiol., 190, 425, 1957.) 



