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According to babycenter.com and countless other pregnancy sites and youtube videos, women should sleep on their left side while pregnant to improve blood flow to the fetus:

Sleeping on your side is best while you're pregnant. In particular, sleeping on your left side may benefit your baby by improving blood flow – and therefore nutrients – to the placenta.

-From babycenter.com

Has definitive research been done proving this to be true?

  • Not an answer to your question, but I was able to dig this up as a possible explanation as to why this would be so: "The heavier uterus rests flat on the inferior vena cava—the main vein just on the right side of your spine that drains the entire lower half of the body. When the vena cava is compressed, much like stepping on a garden hose, flow towards the heart is obstructed and drainage of the lower half of the body becomes sluggish. " --- Full source: babyzone.com/pregnancy/your-body-during-pregnancy/… – Joel Cornett Jun 11 '12 at 2:05
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    And after further digging, here is a 2011 study on the very subject: Association between maternal sleep practices and risk of late stillbirth: a case-control study – Joel Cornett Jun 11 '12 at 2:13
  • @JoelCornett why haven't you written an answer based on that study? It seems to be the best research available on the matter and it's certainly good enough as an answer here imo. – Ian Mar 20 '13 at 12:17
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This study examined whether sleep-position affected still-birth risk: Going to sleep in the supine position is a modifiable risk factor for late pregnancy stillbirth; Findings from the New Zealand multicentre stillbirth case-control study (2017-06-13)

They found that sleeping supine (on one's back) was associated with a much higher risk of late stillbirths:

Supine going-to-sleep position is associated with a 3.7 fold increase in overall late stillbirth risk, independent of other common risk factors.

However, when it came to left versus right, they couldn't draw a conclusion:

In the current study, the risk of late stillbirth was not elevated in women who reported going-to-sleep on their right on the last night and the last week; whereas in our previous study there was a borderline increase in risk with right sided compared with left sided going-to-sleep position on the last night. Further evidence is required to confirm whether right sided going-to-sleep position is associated with similar risk of late stillbirth to left sided going-to-sleep position.

They also looked at previous studies on the physical effects on blood-flow in the different positions.

As discussed in the comments, this paper's results are not strongly powered - they are suggestive, but far from conclusive.

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    Nice find! I made a moderately large edit to extract more information from the study. Please check you are happy with it. – Oddthinking Dec 22 '17 at 2:45
  • Dear statisticians: It seems to me that this study had a number of possible outcomes (and I am wary of the quality of PLOS ONE articles). There seems to be a risk of p-hacking, but I am not skilled enough to make that call. Does it look kosher? – Oddthinking Dec 22 '17 at 2:48
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    @Oddthinking The "Statistical analysis" does seem to indicate that they engaged in some p-hacking by playing around with various models, then presumably calculating the statistics as-appropriate for what they'd regard as their selected model, rather than the meta-model that they implicitly used by trying an ensemble of different modeling techniques. And given the limitations of their data set and the wide-ranging 95%-confidence-interview that has the null hypothesis within a small range distance of the distribution, this study does seem more suggestive than conclusive. – Nat Dec 23 '17 at 21:46
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    @Oddthinking I kinda wonder how we might qualify studies' reliability in general. Like in this answer, the cited study found a very weak conclusion, "Respective incidence rate ratios against controls were 0.64 (95% confidence interval [CI]=0.41-0.99) and 0.89 (95% CI=0.60-1.33).", where the first CI very nearly included the null-hypothesis and the second did include it, even if we're generous in assuming no p-hacking whatsoever. Plus the more recent recommendations call for stronger significance testing, which these results [...] – Nat Dec 23 '17 at 21:55
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    @Oddthinking [...] were too weak to pass for significant under. So, I mean, they're science and science is awesome, making for good contributions toward an answer. However they're certainly not conclusive, either. Dunno how that might be well-expressed in SE.Skeptics answers for a general audience. – Nat Dec 23 '17 at 21:57

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