Burning Question: Why Should Pregnant Women Sleep on the Left Side?

Lylah M. Alphonse, Senior Editor, Yahoo! Shine
Healthy Living

[Every week, Shine finds an answer to one of life's little mysteries. If you've got a burning question you want answered, tweet it to @yahooshine #burningquestions or share it in the comments section.]

It's standard advice as you reach the later stages of pregnancy: Forget how you used to sleep before you were pregnant, and try to make sure that you're sleeping on your left side. But why, exactly?

It has to do with blood flow and the way our circulatory system works, says Dr. Linda Burke-Galloway, an ob-gyn and the author of "The Smart Mother's Guide to a Better Pregnancy." "Our organs and tissues require oxygen to function. Without it, they essentially die," she explains. Oxygen-depleted blood returns to the heart from the lower part of our body via a large blood vessel called the Inferior Vena Cava, or IVC, which is located near the spine. But as your baby grows, your uterus becomes enlarged, and it can press against the IVC, reducing the amount of blood that flows through it.

"When the pregnant uterus squeezes the IVC and reduces cardiac output, a woman might feel dizzy," Burke-Galloway says. "Her blood supply of oxygen is reduced and the unborn baby's is as well."

"The heart can only pump out what comes into it, so less blood into the heart means less blood going out of the heart and the patient feels faint," she adds.

When a very pregnant woman sleeps on her left side, however, the uterus rests against the aorta instead of the IVC, Burke-Galloway explains. The aorta, which brings oxygenated blood from the heart to the other organs, has thicker walls than the IVC and is better able to withstand the pressure from the uterus.

Medical studies have not proven that sleeping on the left side reduces the risk of stillbirth, but doing so is known to increase the flow of blood and nutrients to the growing baby, medical experts say.

If you're not a side-sleeper to begin with, trying to stay on your left may feel strange (keeping your legs bent and putting a pillow between your knees can help). Sleeping on your right side is OK, too -- most people change positions several times while they're asleep anyway -- it's lying flat on your back that has the potential to do the most damage. According to the American Pregnancy organization, in addition to low blood pressure and decreased blood flow for you and your baby, sleeping on your back during when you're heavily pregnant can cause backaches, breathing, digestive system, and hemorrhoids as well.

Not sure whether you need to worry yet? Parenting and pregnancy experts Dr. Bill Sears and Dr. Martha Sears have a sure-fire way to figure it out.

"Realistically, by the time you should not be sleeping on your stomach, you will find it is very uncomfortable to sleep on your stomach; and by the time you should not be sleeping on your back, you will find it is very uncomfortable to sleep on your back," they write Ask Dr. Sears. And, frankly, during that last trimester we were happy to get any sleep at all.

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