Both medical research and clinical observations (primarily FHR monitoring) offer clear evidence as to the benefits associated with left-sided sleep during pregnancy. Even in non-pregnant individuals, sleeping on one’s left side has not only been shown to enhance circulation but also improve lymphatic drainage and gastric function.
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The cost of adopting this healthy habit is literally $0 and yet the benefit can be very real. This article will focus on the physiologic changes associated with lying on your left side and why health care providers, especially during pregnancy, advise lying or sleeping on the left side.
Lastly, I will also mention (as noted above) why left-sided bedrest is the best sleeping position for men and women, barring physical or medical restrictions.
Why is sleeping on your left side beneficial during pregnancy?
To understand why sleeping on your left side is beneficial, it is necessary to review the anatomical location of the great vessel (vein) in the back called the vena cava. This large vessel courses down the back adjacent to the aorta and returns blood to the heart. Due to the specific location of this large vessel (right side along the spine), blood flowing through the vessel is affected by the weight of the pregnant uterus, particularly when lying flat on one’s back. The size and weight of the uterus in the third trimester makes this physiologic observation most pronounced.
The venous system is a low-pressure system, allowing veins to easily be compressed or obstructed by the external pressure from adjacent organs. In contrast, the arterial system is a high-pressure system and for this reason, arterial blood flow is only affected by significant amounts of pressure. It should be no surprise, based on this understanding, why a tourniquet can obstruct blood flow in veins, whereas a high-pressure blood pressure cuff is required to obstruct arterial blood flow.
Furthermore, the fact that the vena cava travels down the right side of the spinal cord is the reason why left-sided positioning is best to optimize perfusion through the vena cava and allow the optimal flow of blood back to the heart and ultimately to the fetal-placental circulation.
Is there clinical proof that sleeping on your left side during pregnancy makes a difference?
Though we know blood flow can be obstructed by the weight of adjacent organs on the vena cava, how do we know that this truly does improve blood flow to the developing pregnancy?
Answer: Research and clinical observations made during fetal heart rate monitoring:
- Research: From a research standpoint, studies have shown that blood flowing back to the heart in pregnant and non-pregnant individuals does improve when sleeping on your left side.
- Clinical Observations: Observations during fetal heart rate monitoring clearly indicate the benefit of left-sided positioning during pregnancy. Any prenatal care provider will relate to what I am going to discuss as this is a common observation and maneuver to restore normal fetal – placental perfusion.
It is not uncommon for a health care provider (doctor/midwife/nurse) to observe a drop in the fetal heart rate during fetal heart rate monitoring. When this occurs, immediate maternal positioning on the left side will most often allow the fetal heart rate to recover. This is the result of facilitating increased blood flow to the fetal-placental circulation, by taking the obstructive weight of the pregnant uterus off the vena cava.
As previously stated, this clinical maneuver will often be all that is necessary to allow the fetal heart rate to return to a normal baseline. Clearly, if this management does not improve the situation, there may be other factors contributing to impaired placental perfusion and further management decisions will be required immediately based on the clinical situation.
Example: The illustration below is what a fetal heart rate tracing will look like when a deceleration of the heart rate is managed with left-sided positioning. This illustration is not being shown to illustrate clinical management. Rather, it is being used to objectively demonstrate a clinical observation resulting from vena cava compression and how sleeping on the left side can favorably impact fetal-placental perfusion.
Illustration of FHR Deceleration response to positioning on left side
So why does this matter?
The answer is simple and can make one wonder why this advice is not discussed more often! Unfortunately, the only time this topic usually comes up is when there are maternal BP concerns, fetal growth concerns or in active labor.
This article is simply intended to show why sleeping on your left side during pregnancy is beneficial.
As mentioned earlier, there are clinical reasons why left-sided positioning during pregnancy is recommended. Left-sided positioning can help manage elevated blood pressure relating to pregnancy. Again this is not just a theoretical benefit as blood pressure clearly can be clinically shown to decrease when a woman with hypertension related to pregnancy lies on her left side. Any woman reading this article who experienced high blood pressure during pregnancy (toxemia/preeclampsia) is very much aware of how important it was to remain on left-sided bedrest whenever possible.
Video Courtesy of HealthTips YouTube Channel
What are some other known benefits of sleeping on your left side?
Our anatomy is the reason for the benefits discussed above. It is important to note that there are some other beneficial physiologic changes that result from sleeping on the left side, even in non-pregnant individuals:
- Stomach: Improves gastric emptying and can reduce heartburn
- Lymphatic System, improves lymph drainage
During pregnancy, always discuss any changes you make to your daily routine or questions that might arise from the information in this article. Only your personal prenatal care provider has the medical history and physical information about your pregnancy that is necessary to make the most appropriate recommendations about your clinical management.
For more information on sleep during pregnancy:
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*** One final note! It should be no surprise that lying flat on your back during pregnancy is best avoided for the reasons mentioned above. Either side is better than lying on your back. As stated in the video, during the later stages of pregnancy, lying on your back can actually lead to maternal dizziness. The information shared in this article is intended to demonstrate objectively why sleeping on your left side can be beneficial even though this topic is not often discussed. Typically the suggestion to rest or sleep on your left side during pregnancy is only discussed during your prenatal care if there are clinical concerns where your provider feels you would benefit from left-sided sleep during pregnancy (ie. fetal growth concerns, high blood pressure).