Why Are Pregnant Women Told to Sleep on Their Left Side?

By Jane Chevalier, LMT

Many pregnant women are told:

“Make sure you sleep on your left side.”

For some, this becomes a casual recommendation. For others, it becomes a source of anxiety, disrupted sleep, or even fear of harming the baby if they wake up on their back or roll to the right side during the night.

So where did this recommendation come from — and what does the research actually show?

The Origin of the “Left Side Only” Recommendation

The recommendation did not originally come from sleep studies.

It came from emergency and hospital obstetric care.

For decades, obstetric providers have used the left lateral position during:

• labor complications

• cesarean anesthesia

• maternal hypotension

• fetal distress

• emergency care situations

• 
  

The reason is physiologic.

In late pregnancy, the growing uterus can compress major blood vessels when a person lies completely flat on their back — specifically the inferior vena cava and sometimes the aorta. This is called aortocaval compression (Kinsella & Lohmann, 1994; Humphries et al., 2019).

Because the inferior vena cava sits slightly to the right side of the spine, turning a pregnant patient onto the left side can improve:

• venous return to the heart

• maternal cardiac output

• maternal blood pressure

• uteroplacental blood flow

  
  

This positioning strategy became standard in obstetric anesthesia and emergency medicine long before sleep-position research existed.

But over time, a hospital positioning intervention gradually became generalized into broad public pregnancy advice:

“Pregnant women should sleep on their left side.”

Those are not the same thing.

What the Research Actually Shows

More recent studies have specifically examined maternal sleep position and late stillbirth risk.

Several large observational studies found an association between:

• going to sleep flat on the back (supine)and

• increased risk of late stillbirth after approximately 28 weeks gestation.

  
  

The Auckland Stillbirth Study (Stacey et al., 2011) was one of the first major studies to identify this association.

The UK MiNESS study later found that women who experienced late stillbirth were more likely to report a supine going-to-sleep position compared with controls (Heazell et al., 2018).

An individual participant data meta-analysis combining multiple studies concluded that:

• supine going-to-sleep position was associated with increased late stillbirth risk

• left and right side sleeping appeared similarly safe (Cronin et al., 2019)

  
  

That distinction matters.

The strongest evidence currently supports:

• side sleeping in late pregnancyrather than

• “left side only” sleeping

Organizations including NICE now recommend:

“Go to sleep on either side.”

Not:

“You must sleep on your left side.”

An Important Detail: “Going-to-Sleep Position”

One important nuance in the research is that many studies examined:

• the position someone fell asleep in

not:

• the exact position they remained in throughout the night

  
  

Pregnant people naturally move during sleep.

Waking up on your back occasionally does not mean something dangerous has happened.

Current public health guidance generally recommends:

• start sleep on either side

• reposition comfortably if needed

• if waking up supine, simply return to a side-lying position

  
  

Why the Left-Side Message Became So Strong

There are several reasons the “left side only” message became culturally dominant.

1. It was based on real physiology

The circulatory effects of late pregnancy are real. Left lateral positioning can reduce vessel compression in some situations.

2. Simple rules spread easily

“Sleep on your left” is easier to remember and teach than:

“Avoid prolonged supine positioning after 28 weeks, but either side is generally acceptable.”

3. Pregnancy advice often becomes risk-focused

Pregnancy culture frequently transforms nuanced physiologic guidance into rigid behavioral rules.

What begins as:

“this may reduce risk”

can become:

“if you don’t do this perfectly, you could harm your baby.”

That is a very different message psychologically.

The Problem With Fear-Based Sleep Messaging

Many pregnant women already struggle with:

• hip pain

• pelvic discomfort

• reflux

• insomnia

• frequent waking

• nervous system hypervigilance

  
  

Adding fear around sleep position can worsen:

• anxiety

• sleep quality

• muscular tension

• obsessive sleep monitoring

  
  

And poor sleep itself has physiologic consequences for maternal health.

There is a difference between:

• supportive physiologic guidanceand

• creating fear around body position all night long

  
  

So What Should Pregnant Women Actually Do?

Based on current evidence:

• side sleeping in late pregnancy is generally recommended

• either side appears acceptable

• the left side is not clearly proven superior to the right side for most healthy pregnancies

• waking up on your back occasionally is common and not considered an emergency

  
  

The goal is not perfect positioning.

The goal is supporting comfortable, restorative sleep while reducing prolonged fully supine positioning in late pregnancy.

Final Thoughts

The “sleep on your left side” recommendation came from legitimate obstetric physiology and emergency-care practice. But over time, it became simplified into a rigid pregnancy rule that often exceeds what the evidence actually supports.

Pregnancy care should support both:

• physiologic safetyand

• nervous system safety

Those are not separate conversations.

References

Cronin RS, Li M, Thompson JMD, et al. (2019). Maternal going-to-sleep position and late pregnancy stillbirth: Individual participant data meta-analysis. EClinicalMedicine, 10, 49–57.

Heazell AEP, Li M, Budd J, et al. (2018). Association between maternal sleep practices and late stillbirth – findings from a stillbirth case-control study. BJOG, 125(2), 254–262.

Humphries A, Mirjalili SA, Tarr GP, et al. (2019). The effect of supine positioning on maternal hemodynamics during late pregnancy. Journal of Maternal-Fetal & Neonatal Medicine, 32(23), 3923–3930.

Kinsella SM, Lohmann G. (1994). Supine hypotensive syndrome. Obstetrical & Gynecological Survey, 49(7), 497–508.

Stacey T, Thompson JMD, Mitchell EA, et al. (2011). Association between maternal sleep practices and risk of late stillbirth: A case-control study. BMJ, 342:d3403.