A quantitative study of follicular development indicates that there is no cyclic activity during pregnancy in the rat comparable to the changes of the estrous cycle. The number of antral follicles is relatively constant throughout pregnancy. Follicles larger than 600 μ in diameter do not appear until the last day of pregnancy (day 22). Ovulation could not be induced with HCG until day 21 of pregnancy in the rat in contrast to the pregnant hamster in which ovulation and even superovulation can be readily produced. It appears likely that the refractoriness to ovulation in the pregnant rat is due to the physiologic immaturity of the vesicular follicles. There is a 3-fold increase in pituitary content of LH between days 1 and 8 of pregnancy and thereafter the level remains relatively constant until day 22. During the last week of gestation there is a 4-fold increase in pituitary FSH so that the FSH:LH ratio rises to 9:1. During the first half of pregnancy the FSH:LH ratio is 4:1. The presence of vesicular fol...
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Gilbert S. Greenwald (1966) studied this question.