Maternal blood flow to the uterus increases from about 50 ml/min at 3 mos of gestation to 500-750 ml/min at term. Thus, an increasing fraction of the maternal cardiac output perfuses the uterus as development progresses. In order to accomplish this and still adequately perfuse the maternal tissues, maternal cardiac output increases from 4,500 to 6,000 ml/min. This is accomplished by increases in both heart rate and in stroke volume.
The placenta contains 60 fetal cotyledons and 20 maternal lobes formed by septa. At term there are some 50,000,000 fetal villous capillaries.
Maternal blood flow in the intervillous space (blood pool) is 65-120 ml/100 g/min, while fetal villous flow is 90-190 ml/100 g/min. Thus, maternal and fetal blood flows are fairly evenly matched, i.e. approximately 1 for 1.
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