Sleep On Side (SOS)

Sleep On Side (SOS) is the NHS and ACOG-recommended sleeping position from 28 weeks of pregnancy. Both bodies advise sleeping on your left side rather than your back from 28 weeks onwards.

Why left-side sleeping is recommended from 28 weeks

When you sleep on your back in late pregnancy, the weight of the uterus can compress the inferior vena cava — the major vein that returns blood from the lower body to the heart. This reduces blood flow and can cause reduced foetal movement, lightheadedness, and nausea in the mother.

A 2019 study published in The Lancet (Heazell et al., 2019) found an association between back sleeping in the third trimester and increased risk of stillbirth. The absolute risk remains low, but the NHS and ACOG guidance changed in response to this evidence: go to sleep on your left side.

The key word is “go to sleep.” Waking on your back is normal and not cause for alarm. The guidance is about your starting position, not policing every movement during the night.

Why left over right

Left-side sleeping is specifically recommended because the inferior vena cava runs slightly to the right of the spine. Left-side sleeping keeps the uterus from compressing it. Right-side sleeping is considerably better than back sleeping. If you wake on your right side, do not panic — move to your left and return to sleep.

When to start

The NHS guidance says “from 28 weeks.” Research suggests earlier side-sleeping is no harm and may help with:

• Building the habit before the bump is large enough to make it uncomfortable
• First-trimester insomnia management (side sleeping is generally well-tolerated)
• Round ligament pain management through supported positioning

How pregnancy pillows support the SOS guidance

Pregnancy pillows exist primarily to make SOS-compliant sleep comfortable enough to sustain throughout the night. Specifically:

• C-shape (Snoogle): supports bump, back, and knees on the left side. The back arm prevents rolling to your back.
• U-shape (Momcozy): supports both sides simultaneously. If you roll from left to right (and right is fine), support is maintained.
• Wedge (Boppy): supports the bump only. Does not prevent back-rolling.

Sources

• NHS: “Sleeping in pregnancy” — nhs.uk/pregnancy/related-conditions/common-symptoms/sleeping-problems/
• Heazell et al. (2019), The Lancet: “Associations between maternal sleep practices and late stillbirth”
• ACOG Practice Bulletin on Sleep Disorders in Pregnancy

Always consult your midwife or OB about sleep positioning recommendations during pregnancy, especially if you have complications such as a low-lying placenta or pre-eclampsia.