THE HORMONES IN PREGNANCY 



the blood of the pregnant mare there is a gonadotrophic hor- 

 mone which also has a stimulating effect on the ovaries of test 

 animals, such as rats, differing considerably, however, in de- 

 tail. This does not get into the urine in significant amounts, 

 which means that it must be different chemically from the 

 human hormone of similar action. 



We do not know why these substances can be found in some 

 species of animals and not in others, nor do we know what 

 function they perform in pregnancy. The whole series of 

 pituitary and pituitary-like hormones has been extremely 

 difficult to investigate chemically because the substances are 

 proteins and they defy purification. The ovaries of the rat 

 and the rabbit can distinguish them better than the chemist. 

 For the present we must content ourselves with being grateful 

 for the pregnancy test and await the day when these trouble- 

 some substances yield themselves to chemical isolation. 



As mentioned previously (Chapter IV) the urine of preg- 

 nant women contains relatively large amounts of estrogenic 

 substances, which increase as pregnancy advances and dis- 

 appear after parturition. These substances have been found 

 in the urine of several other species during pregnancy. The 

 human placenta also contains large amounts of estrogenic 

 hormones, chiefly estriol, and is almost certainly the source 

 of those which appear in the urine. As mentioned in Chapter 

 IV, when the ovaries are removed during pregnancy, estro- 

 gens continue to be excreted in the urine, a fact which proves 

 that some other source exists, and this can hardly be anything 

 else than the placenta. A similar situation, produced experi- 

 mentally in the monkey, has been studied very carefully and 

 reported by R. L. Dorfman and Gertrude Van Wagenen of 

 Yale Medical School. 



There are several possible ways in which the production of 

 estrogens by the placenta may be useful. It has been suggested 

 that these hormones are needed, in larger amounts than the 

 ovaries can provide (a) to promote the growth of the uterus 



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