Both medical research and clinical observations (primarily FHR monitoring) show the benefits of left-sided sleep during pregnancy.
Even in non-pregnant individuals, sleeping on one’s left side has been shown to enhance circulation as well as 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 left-sided sleep during pregnancy and why health care providers, especially during pregnancy, advise lying or sleeping on your 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 left-sided sleep during pregnancy considered the best sleeping position?
To understand why left-sided sleep during pregnancy 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 sleep during pregnancy 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 can be beneficial?
Yes, there is clinical proof. We know blood flow to the placenta is decreased by the weight of the pregnancy on the vena cava* based on 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 demonstrate the benefit of left-sided maternal positioning. Any prenatal care provider can relate to what I am going to discuss as this is a common observation and maneuver we all have used to improve fetal-placental blood flow.
The Vena Cava is the large vein that travels down your back adjacent to the aorta and spine. This blood vessel returns blood to the heart. The blood pressure in the Vena Cava is low, so it can be compressed when pressure is put on it by adjacent structures.
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.
This maneuver will often be all that is necessary to allow the fetal heart rate to return to a normal.
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 illustrates what a fetal heart rate tracing will look like when decelerations of the fetal heart rate are 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 responding to left-sided maternal positioning
So why is sleeping on your left side best during pregnancy?
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 BP concerns, fetal growth concerns or during active labor when there are changes in the fetal heart rate tracing.
This article is simply intended to show why left-sided sleep during pregnancy is beneficial. If you are reading this, hopefully, you will find it helpful.
Perhaps the day will come when left-sided bedrest is discussed in more detail and suggestions such as using a pregnancy pillow (see below) become a routine recommendation during one’s prenatal care.
There are clinical reasons why left-sided maternal positioning helps 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 has or had high blood pressure during pregnancy (toxemia/preeclampsia) knows how important it is to rest on their left-side whenever possible.
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
If you find it difficult to sleep on your left side, use a pregnancy pillow. The comfort alone will leave you wondering how you ever got along without one!
Pregnancy pillows offer a number of benefits. It can relieve the discomfort many pregnant women experience in their lower back and pelvis.
It also can be therapeutic for musculoskeletal conditions that may have been present prior to pregnancy such as sciatica and chronic low back pain.
The illustration below shows how the pillow can help to maintain alignment of the neck, spine, pelvis and lower extremities.
During pregnancy, always discuss any changes you make to your daily routine, such as using a pregnancy pillow, with your prenatal care provider.
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 about sleep during pregnancy, check out some of our other articles:
*** One final note! Lying flat on your back during pregnancy should be 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).