24 In an elegant study, Rasanen et al. showed how reactivity in the pulmonary vascular bed increased in the third trimester.14 While fetuses at gestational age 20e26 weeks showed no changes during maternal hyperoxygenation, fetuses at 31e36 weeks had lower impedance in the pulmonary arteries assessed by the pulsatility index, and increased pulmonary blood ow. Correspondingly, the blood ow in the ductus arteriosus was reduced.

Brain circulation Differences in circulation physiology between animal experiments and human fetuses are likely to be greatest when concerning the brain, as the human brain is relatively larger than in other species. In a study of human previable fetuses weighing 12e272 g (probably corresponding to 10e 20 weeks of gestation), it was found that the brain received approximately 15% of the systemic venous return (equal to the CCO less the pulmonary circuit).33 The proportion directed to the brain increased with low arterial pH, increased pCO2 and reduced placental perfusion. A study of the primate Macaca mulatta at an advanced stage of gestation found that 16% of the CCO was distributed to the brain, and this fraction increased to 31% during hypoxic challenge.21 Both of these studies

Clinical obstetrics has taken advantage of such brain-sparing mechanisms, and uses the increased diastolic blood velocity recorded in the middle cerebral artery as a marker of compensatory redistribution of blood to the brain.57

Fetoplacental circulation In the fetal sheep, 45% of the CCO is directed to the umbilical arteries and placenta.58 This percentage is less in exteriorized human fetuses, but it increases from 17% at 10 weeks to 33% at 20 weeks of gestation.33 These results overestimate the placental fraction as the CCO calculation was based on systemic venous return, not including the pulmonary venous return. Secondly, the measurements were not performed under strict physiological conditions. Doppler studies of low-risk pregnancies have found similar results; one-third of the fetal CCO is directed to the placenta at 20e32 weeks of gestation,59,60 but this decreases to approximately one-fth beyond 32 weeks of gestation.60