We have examined the relationships among fetal maturity, adrenal steroidogenesis, and initiation of parturition using embryo transfer procedures. Ovine embryos, genetically coded for relatively short [145.5 ± 1.4 days; Finnish Landrace (Finn)] and long [150.7 ± 1.3 days; Rambouillet (Ra)] gestational periods, were transferred to a common recipient, thereby facilitating comparative endocrine studies in genetically dissimilar fetuses with a common intrauterine environment. Maternal and fetal plasma samples were obtained daily from chronic fetal lamb preparations in six mixed breed (MB), three Finn, and three Ra single breed (SB) pregnancies during the last trimester and assayed for cortisol, desoxycortisol, and progesterone by RIA. Mean ± SEM gestational lengths of the forenamed pregnancies were 141.5 ± 0.7, 141.7 ± 0.7, and 149.3 ± 0.9 days, respectively. Time-trend analysis of C-21 steroid data showed an increase in Finn and Ra SB fetal plasma cortisol of 1.0 ng/ml·day at 9.03 ±1.0 and 9.58 ±1.10 days prepartum, respectively. Significant (P ≤ 0.05) differences in the number of days of incremental change (T*) were noted between Finn (10.56 ± 0.67) and Ra (4.82 ± 1.08) siblings in MB pregnancies at increment rates of 1.0 ng/ml·day. Similarly, significant (P ≤ 0.05) differences in T* values were observed between Finn (8.31 ± 0.38 days) and RA (4.63 ± 1.0 days) siblings at 2.0 ng/ml·day. While Finn and Ra SB fetal desoxycortisol T* values were similar at 0.5 and 0.1 ng/ml·day, significant differences (P ≤ 0.05) were noted in Finn and Ra cofetuses in the MB pregnancy. Prepartal changes in maternal and fetal progesterone concentrations were correlated, and a direct parallelism was noted between fetal sibling progesterone concentrations. These results are consistent with the view that the fetal genome has a principal role in the timing of parturition.
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