Increased myometrial contracture frequency at 96-140 days accelerates fetal cardiovascular maturation

Fetal cardiovascular responses to an altered intrauterine environment of in creased myometrial contractures induced by oxytocin (OT) pulses to the ewe over the final 50 days of gestation were studied in chronically instrumente d sheep. Ewes received saline (Cntl) or long-term OT treatment (LTOT, 600 m u U . kg(-1) . min(-1) in 5-min pulses every 20 min) from 96 days gestation al age. Fetal baroreflex responses to sodium nitroprusside (SNP) and phenyl ephrine (PE) were studied at 133 days gestation. OT increased contractures in LTOT ewes. Fetal blood pressure (FBP) was higher, and fetal heart rate ( FHR) and slope of daily change in FBP and FHR were lower in LTOT fetuses. F etal SNP-induced hypotension resulted in a narrow R-R interval variation ra nge in LTOT fetuses; Cntl fetuses showed early breakdown in compensation. B aroreflex response slope during PE-induced fetal hypertension was lower in LTOT than in Cntl fetuses. Although the cortisol-to-ACTH ratio was lower in LTOT fetuses, fetal plasma ACTH and cortisol changes were similar in contr ol and LTOT fetuses. We hypothesize that contracture-induced alterations in the intrauterine environment accelerate fetal cardiovascular development t hrough mild hypoxemia, repetitive fetal pituitary-adrenal stimulation, and/ or physical stimulation.