Blood Volume Changes

Something interesting that I’ve noticed this week is that I’m definitely breathing at a faster respiratory rate (RR) with limited activity although my HR seems to be normal. And more than occasionally when I get up from sitting for more than 10 minutes I get the borderline blackout symptoms. But from what I’ve read, my blood volume may already be starting to increase?  Here’s some really interesting information on blood volume, cardiovascular and pulmonary changes during pregnancy.

Did you know, that the average pregnant woman’s plasma volume will increase by 1250 -1600 ml? That’s about the same as a 50% increase! Research suggests that this increase is very rapid until about 20 weeks and then although it continues to rise, the rate it rises slows, topping off around 32 weeks. Although, some studies show that by weeks 6-12 you may already have increased your blood volume by 10-15%. Along with this, red blood cell mass increases by about 250 ml or 18% throughout pregnancy. Maybe even more if you are taking an iron supplement. Your red blood cells tend to rise in a linear fashion from the end of the 1st trimester until birth. Interestingly, this drops your hematocrit levels from about 40 to 33 during the last trimester (if you’re having a blood panel screen and are curious as to the changes in these numbers). The importance of increase in maternal blood volume is two fold. First, it helps to support the exchange of oxygen, CO2 and nutrients to and from the baby, and secondly, it reduces the impact of maternal blood loss at delivery.

Along with the increase in blood volume, your cardiac output increases by 30-40% by the end of the first trimester, meaning that your heart is working 30-40% harder all of the time. During labor, much of the blood that was being circulated to the baby gets shunted back into your venous system to compensate for blood loss. This further increases your cardiac output to about 50% of pre-pregnancy values.

Your lungs also feel the changes in your cardiovascular system. You may notice an increase up to 50% of your respiratory rate at rest and with activity as well as a 15-20% increase in oxygen consumption. Furthermore, you may see a 30-40% increase in tidal volume which is the amount of air that is inhaled and exhaled during a normal breath. You may also see a 20% decrease in functional reserve capacity or the amount of air remaining in your lungs after a normal expiration. Basically, what all of this means is that you’re likely breathing faster and harder without any increase in workload.

Another symptom you may notice due to changes in blood volume is a slight decrease in blood pressure between 12-26 weeks.


So what does all of that really mean? Well, the increase in blood volume means that you may experience; swelling, lower tolerance to hot temperatures (even when you’re working out), dizziness, fatigue, varicose and spider veins, increased sweating, increased libido, and hemorrhoids. And increased cardiac output and pulmonary changes are to thank for that increased shortness of breath you may have noticed when you’re going about your daily routine. For example, I am quickly out of breath when I climb a flight of stairs now, which was certainly not the case before. And of course, a change in blood pressure due to the influx of blood volume is totally normal. Just be careful when moving from lying down to standing. Take your time and don’t forget to breath while you’re moving!


For those of you who are even a bit nerdier (like me) and would like a more in-depth look at this info, here are a bunch of charts that summarize all of the physiological changes that are taking place. And, here (Physio changes, book chapter) is a book chapter that has some really detailed info if you’re still hungry for more! Enjoy!

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Binkley, Helen M. et. al. “Land-Based Exercise During Pregnancy.” International Journal of Childbirth Education; 2015: 30 (3).
Newton, Edward R and Linda May. “Adaptation of Maternal-Fetal Physiology to Exercise in Pregnancy: The Basis of Guidelines for Physical Activity in Pregnancy.” Clinical Medicine Insights: Women’s Health; 2017: 1-12.
Ouzounian, Joseph G. and Uri Elkayam. “Physiologic Changes During Normal Pregnnacy and Delivery.” Cardiol Clin; 2012: 30.
Torgersen, Col Keiko and Carol Curran. “A Systematic Approach to the Physiologic Adaptations of Pregnancy.” Critical Care Nursing Quarterly; 2006: 29.


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