A Quantitative Relation between the Experimentally Determined Number of Conceptuses and Corpus Luteum Activity in the Pregnant Rat*

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Abstract
The relation between the number of live conceptuses in the uterus in pregnant rats bearing a full complement of corpora lutea (CL) and the growth of the CL and their rate of progesterone secretion after day 12 was determined by aspirating all but the desired number of conceptuses on day 7 of pregnancy (day 1 = insemination). In one series of rats, bearing 0, 1, 2, 3, 4, 5–7, or 10 or more conceptuses, the weights of the CL and the serum progesterone levels were measured on days 12 and 15; in another, consisting of rats with 0, 1, or 10 or more conceptuses, the serum progesterone level was measured on days 12, 15, 17, 19, and 21. There was a direct relation between the number of conceptuses, the size of the CL, and the serum progesterone level on day 15. The maximum normal values for both parameters of CL activity were found in rats with 4 or 5 or more conceptuses. In the rats bearing 10 live conceptuses, the serum progesterone levels rose and fell between days 12–21, very much as has been described for normal pregnant rats. In the rats with one conceptus, however, the serum progesterone levels remained below the day 12 values throughout the period of observation. The results suggest that a placental hormone stimulates the CL to grow and increase their rate of progesterone secretion after day 12 by stimulating the production of intraluteal estrogen. This hormone is apparently secreted by each placenta at a rate unaffected by that of the other placentas. The reasons for this hormone being different from the rat's placental lactogen were discussed.