Researchers measured growth hormone patterns in pregnant rats and found that levels were roughly double those of non-pregnant rats during late pregnancy, with an especially dramatic fourfold increase on day 20 of gestation. The elevated growth hormone came from the pituitary gland, as confirmed by the fact that removing the pituitary eliminated all detectable growth hormone. Pregnant rats also responded consistently to growth hormone-releasing factor injections, showing sustained pituitary responsiveness throughout pregnancy.
Carlsson, L; Edén, S; Jansson, J O