Maternal hemodynamics and uteroplacental blood flow throughout gestation in conscious rats.

The present studies were conducted to develop experimental methods for obtaining cardiovascular measurements in conscious, pregnant rats. Methods were then used to establish key quantitative measurements of (a) placental and fetal development, (b) maternal hemodynamics and (c) uteroplacental blood flow throughout gestation. A progressive and significant enhancement in fetal weight occurs during the later one-third of gestation, commencing at day 15 of pregnancy. Fetal mass increases from 100 mg to 6,500 mg over this time period. Placental weight increases concurrently; however, over the last one-third of gestation, fetal growth and development outstrips placental augmentation such that each placental unit supports 10 fetal units near-term. As late gestation proceeds, a progressive reduction in mean arterial blood pressure is accompanied by enhancement in cardiac output. Total peripheral resistance is significantly reduced. Starting at day 15 of gestation, progressive and significant enhancement in blood flow to uterine tissue occurs. Moreover, significantly greater percentage of cardiac output is directed toward uterine tissue, especially near term. Percentage uterine blood flow directed to placenta is less than 10% at day 15 gestation, but placental perfusion is preferentially enhanced as late gestation continues. Near-term, 90% of uterine blood flow perfuses the placenta. Blood flows to nonreproductive visceral organs (kidney, liver, gut and spleen) and skeletal muscles (gastrocnemius, plantaris, tibialis anterior and soleus) are unaltered by pregnancy and gestation. Therefore, late gestational rat fetal development is supported by cardiovascular adaptations, including specific regional control mechanisms, qualitatively similar to those occurring during human gestation.