Heart Rate and other Cardiovascular Changes During Pregnancy The pregnant woman’s body undergoes massive physiological changes during pregnancy and one of the most fascinating systems to talk about is the cardiovascular system. By about 16 weeks, resting cardiac output (the amount of blood your heart can pump) increases from about 5 to 7 liters of blood per minute. That’s nearly a 50% increase! Just imagine the amount of energy and resources needed to make this reality: hello, first trimester fatigue, shortness of breath, lightheadedness, increased appetite, etc.
Don’t get discouraged when these side effects hit--you are working in accordance with your design to do one of the most miraculous tasks a human can do. Both stroke volume (SV) and heart rate (HR) contribute to this massive increase in cardiac output (Cardiac Output = SV * HR). Stroke volume is how much blood leaves the heart each beat and HR is the frequency of those beats. So, if we were to apply a bit of logic to this equation, SV and HR have to increase in order to get a greater cardiac output. We see this happen through a number of mechanisms: increased volume of the heart chambers, increased blood volume (more red blood cells, white blood cells, platelets, and plasma), and new growth of blood vessels (angiogenesis). Not coincidentally, these are the same adaptations we see when an untrained person begins a fitness program because… Pregnancy is the ultimate expression of fitness. So what does this mean for mom and baby? It means that mom can supply more nutrients to her growing baby, accommodate her increased blood volume, more efficiently regulate the body temperature of her and baby, increase the availability and usage of oxygen at the cellular level, increase her metabolic capacity, and increase her resiliency to exercise-induced stress (or perhaps labor). In his book, Exercising Through Your Pregnancy, Dr. James Clapp notes that when a trained pregnant woman continues to train during her pregnancy, these physiological benefits that we see are at least additive, if not greater. That’s why we see some pregnant women with surprisingly low resting heart rates--their body has adapted to the state of a highly trained athlete. Their hearts can more efficiently meet the needs of their system.
For baby, all these cardiovascular adaptations serve to nourish baby better. Through a regular fitness practice, baby also gains the incredible benefit of being more resilient through stress and stimuli (a handy skill for birth and beyond)!
The point is that during pregnancy, you are more fit than you may feel. Your body is incredible and so are you.