Ask almost any pregnant or recently pregnant woman and most will tell you that a pregnant woman should never sleep on her back, and is "supposed to" sleep on her left side at night. In my practice, I hear this from all sorts of pregnant women -- often as early as 12 weeks. Unfortunately, lying on your left side all night every night while you are already uncomfortable can be extremely frustrating for many women who have enough difficulty trying to get the rest their body needs (frequent bathroom trips, heartburn, fetal movements, etc.). Many women will start describing their aches and pains, and inevitably, say, “And, because I can only sleep on my left side…” She’ll then proceed to tell me how her limited sleeping positions are actually exacerbating her pain and discomforts.
A quick survey of pregnancy guides, and an online search on “pregnancy sleeping positions” yields a solid consensus, “left side-lying is the best sleeping position in pregnancy.” These books and sites proceed to describe a host of reasons why “left side” is best. The most wide spread logic given is that this position interferes the least with venous flow, though several also indicate that this position is ideal for the liver and kidneys. Despite the ever-presence of this information, when most of my clients actively engage their OB or midwife, they are told that if they are healthy, they should sleep however they are most comfortable -- their bodies will actually let them know when it is time to change positions. Many OBs encourage women not to freak out if they wake up in the middle of the night flat on their backs; "waking up" is their body's way of telling them it's time to roll over.
In my own training, I have spent a considerable amount of time studying safe positioning for women at various stages of pregnancy, and simultaneously, learning ways to always make sure pregnant women are comfortable. The safety concerns were always most restrictive for certain pregnancy complications and for women closer to term. But my job is to make pregnant women comfortable! So, over the next several months, I will be delving into the medical literature to add some more complexity to this issue.
Off Your Back!Most sources that recommend a left side-lying position are drawing on information gleaned from researchers in the mid-20th century(Kerr, et al) who found that when very pregnant women lay flat on their backs, the weight of the pregnant uterus partially compresses the inferior vena cava, and others soon after found that the aorta as well was partially occluded (Bieniartz, et al). These studies have found that in some women (approx 2 to 4% of third trimester women), the weight of the uterus on the inferior vena cava and aorta can cause blood to pool in the legs, decreasing blood return to the heart and reducing maternal cardiac output. This may then result in a drop in mom's blood pressure and what has come to be known as "supine hypotensive syndrome" -- which often results in a feeling of faintness, and very rarely actual fainting may occur. Researchers have found that while lying flat on the back leads to vena caval occlusion in all full-term pregnant women, only 5-11 % experience symptoms of hypotension (Kerr, 1965; Kerr, Scott, & Samuel, 1964), and another review of the literature estimates only 2% to 4% of women have significant compression (Farine, 2007). It has been theorized that women who do not experience these symptoms may have a supplementary paravertebral collateral circulation that efficiently returns blood from the pelvis and legs (Kerr et al., 1964; Kinsella & Lohmann, 1994). This supplementary circulatory pathway may not be adequate in those who become symptomatic when lying flat.
Left is Best.Why left side-lying? Research indicates that while lying on her left side (as compared to right side-lying and flat on her back), a pregnant woman's cardiac output is optimized, as is the oxygenation of her blood. All this suggests to the researchers that any aortocaval compression is less in left side-lying. I am still looking for a reference for this, but I had always learned that it was because of anatomy. The vena cava runs lateral to the midline of the body on the right side, and the aorta to the left. Veins are much less rigid than arteries (particularly the aorta), and would be more prone to compression from a heavy, pregnant uterus than arteries.
Yet, neither the American College of Obstetricians and Gynecologists, nor the Society of Obstetricans and Gynaecologists of Canada make any recommendations on sleeping positioning during pregnancy, and most of my clients' obstetricians when approached around this issue, will tell their patients that a healthy woman doesn't need to worry about sleep positioning; she needs to focus on getting comfortable and listening to her body (ie., if she feels dizzy or faint while lying flat on her back, she shouldn't lie on her back).
I was amused to come across a 2007 "Commentary" in the JOGCby Farine and Seaward that reviews the medical literature on sleep positioning in pregnancy. These two MDs found that an increasing number of their pregnant clients were asking if they must sleep on their left sides. They write, "Finding that this relatively unimportant topic was increasingly encroaching upon our time with patients, we did some research." They proceed to discover that the internet was the major source of information on sleeping positions for their moms-to-be, and all but one site in the top search results encouraged women to sleep on their left sides (and from some of the sites I read, they also told women that they would be cutting off their baby's oxygen flow). Only a few of the sites distinguished between stage of pregnancy, or gave any guidelines as to whether a few minutes on your back would be safe. Farine and Seaward conclude:
"In conclusion, advising women to sleep or lie exclusively on the left side is not practical and irrelevant to the vast majority of patients. Instead, women should be told that a small minority of pregnant women feel faint when lying flat. Women can easily determine whether lying flat has this effect on them, and most will adopt a comfortable position that is likely to be a left supine position or variant thereof. Since healthy pregnant women often require more pillows (...and) since finding a comfortable position in bed in late pregnancy is not easy, physicians should refrain from providing impractical advice"(842).
While research does seem to demonstrate that sleeping positions do impact some women's blood pressure and cardiac output, this does not demonstrate fetal compromise in healthy pregnancies. Farine and Seaward wryly note, "If lying prone had been detrimental to a normal pregnancy, the species would long ago have ceased to exist."
Conclusion: Don't believe everything you read online! While sleeping positioning can have an impact on both mom and baby, the reality is that most doctors and midwives tell women that any comfortable sleeping position will work; strict left side-lying is not medically necessary for most women. These websites and books are being overly cautious. The women who do need to be concerned with their particular positioning are women who are experiencing certain complications. Most women in healthy pregnancies should find positions that are comfortable and do their best to get a good night's rest. Some women will be advised by their caretaker that they should restrict their sleeping positions to side-lying or strictly left side-lying only -- often there are concerns about the functioning of the placenta or mom-to-be's blood pressure. If you are not sure about your own situation, call your caregiver!
2018 Update: This has just become a muddier issue over the past few years with a few studies indicating that left side sleeping results a lower rate of late-term stillborn births. These are not conclusive at all and heavily criticized. The conclusion remains, talk to your provider about your own risk factors!!!!