The Physiological Role of Relaxin in the Pregnant Rat. I. The Influence of Relaxin on Parturition*

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Abstract
Length of gestation, duration of labor and delivery and fetal survival were determined in control intact pregnant rats (group C). Pregnant rats were bilaterally ovariectomized on day 9 and given progesterone (P) implants and, in addition, one of the following injection regimens; estrogen (E; group OPE), E and porcine relaxin (R; group OPER), or porcine R (group OPR). Hormone treatments were given in doses designed to produce serum levels of these hormones similar to those observed in intact pregnant rats. The P implants were removed during the evening of day 21 to mimic the decline in serum P levels that normally occurs as a result of luteolysis. Animals in groups OPE and OPR exhibited significantly prolonged duration of labor and delivery, and reduced fetal survival compared with controls. Group OPER animals exhibited slightly but not significantly shorter length of gestation, similar duration of labor and delivery, and similar rate of fetal survival compared to control values. Group OPER animals showed normal maternal behavior and were able to suckle their young.