Maternal Physiology 101

It’s time to get your geek on and learn a little physiology! Today’s post is about some of the ways your body adapts to pregnancy. Literally every organ system changes to support the growth of a baby. These changes are truly remarkable. Some of the most dramatic changes occur in the cardiovascular system. Here are the highlights.

Pregnancy Physiology | CU OB/GYN | Photo of pregnant woman in mirror

Your blood volume expands by 33 percent

The next time you are at the grocery store, take this blog post with you and head for the dairy section. Hopefully, this will help you visualize the volumes we are talking about. Most estimates place the normal blood volume of an adult woman at around 4.5 – 5.0 liters. So start your in-store demonstration by pulling 4 or 5 quarts of milk out of the refrigerated cases and load them into your cart. This approximates your blood volume before your pregnancy. Now, add another 3 pints of milk, around 1.5 liters. This is how much your circulating blood volume expands during a normal pregnancy. Take a moment to appreciate the weight of this additional volume. If your blood were purely water (which it isn’t), this expansion would weigh around 3.3 pounds. Pretty impressive. Conclude this lesson by making a purchasing decision about the milk. If you don’t want actually want it, put it back and move on before the manager asks what you are doing.

Your cardiac output increases by 30 to 50 percent

This adaptation to pregnancy is also pretty impressive. But what on earth is cardiac output? The simplest explanation is that cardiac output is a measurement of blood flow. This flow is typically expressed in units of milliliters per minute. Determinants of cardiac output include heart rate and stroke volume. Heart rate, the speed at which the heart pumps blood, is what we are measuring when we count our pulse. It is usually recorded as beats per minute. A normal heart rate in an adult is 60 – 100 beats per minute. Stroke volume is a measure of how much blood is pumped out of the heart with each beat. In a healthy, non-pregnant woman, a normal stroke volume would be around 60 milliliters per beat. When you do the math, then, normal cardiac output falls in the range of 3600 to 6000 cc per minute in a healthy, non-pregnant woman. If the manager hasn’t run you out of the store already, go back and see how much fluid 3 – 6 quarts of milk actually is. When you add pregnancy to the mix, these numbers increase. One study showed increases to 7.3 liters per minute late in pregnancy, just as a result of the pregnancy itself. The increase in cardiac output is accomplished by increases in both heart rate and stroke volume.

Where does the extra blood flow go?

As you might imagine, much of the increased flow goes to support the uterus and the developing pregnancy. In the non-pregnant state, the uterus receives 2 – 3 percent of a woman’s blood flow. The pregnant uterus at term, on the other hand, received around 17 percent of the output. This is a dramatic increase.

Exercise during pregnancy

With all of these changes occurring at baseline, are you actually supposed to exercise during your pregnancy?  Well, the simple answer is YES. But that is another topic for another day.