Fetal cardiac output increases steadily (geometrically) with development (Table 3.01, Figure 3.01). Normalized to body weight, it ranges from 235-525 ml/kg/min. Adjustments in cardiac output are brought about mainly by changes in heart rate.
Heart rate increases from 65 beats/min in the 15 mm embryo to 175 beats/min in the 9 week fetus. From midway through gestation until term the heart rate ranges from 135-155 beats/min. The vagal pathway and chemoreceptors on large blood vessels are present by 6-8 weeks of development. Bradycardia is the normal response to maternal hypoxia.
Arterial blood pressure shows a steady rise with later fetal growth (Figure 3.02), with values of 30 mmHg up to 50 mmHg in the umbilical arteries, and values of 6 mmHg up to 16 mmHg in the umbilical vein. Blood pressures in the pulmonary artery and aorta rise progressively, reaching 50-80 mmHg near term.
To review blood pressure measurement
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