Is it safe to lie on your back during pregnancy?
If you have asked this question, you have probably already received at least two different answers. Your doctor or midwife likely said from 12 weeks. Somewhere in the fitness and health industry, you may have heard anything from 16 to 28 weeks. That is a big range, and it is reasonable to want to understand why.
Here is what is actually going on.
Why it matters at all
As your uterus grows, its weight can press on a large vein that runs behind it called the inferior vena cava. This vein carries blood from your lower body back to your heart. When it gets compressed, less blood returns to your heart, your blood pressure can drop, and blood flow to your uterus is reduced.
Before pregnancy, your uterus receives around 100ml of blood per minute. By the end of pregnancy that rises to around 700ml per minute. That blood flow cannot regulate itself. It depends on your position and blood pressure staying stable. This is why lying flat becomes more of a consideration as your pregnancy progresses.
What it can feel like
Some women feel nothing at all. Others feel dizzy, nauseous, sweaty or lightheaded when they lie flat. Around 15% of women experience a significant drop in blood pressure in this position.
What makes this tricky is that compression of the vena cava does not always produce obvious symptoms. Some women feel it immediately. Others feel nothing at all, even when blood flow is being affected. There is no reliable way to tell in the moment which category you fall into.
A sudden episode of dizziness or fainting during exercise is frightening. It can put you off exercising altogether and lead to unnecessary medical investigations. For women who fall, the anxiety can be significant. Many will worry about what the fall has done to their baby. In most cases the baby will be fine, but knowing that does not always silence the what if. For many women that question sits with them until their baby is born. That is a long time to carry that worry.
Why WHEN recommends from 12 weeks
WHEN aligns with RANZCOG and ACOG in recommending you avoid lying flat from 12 weeks. This is not because the evidence of fetal harm at 12 weeks is definitive. The research in this area is limited and much of the guidance across professional bodies is based on expert opinion rather than high quality direct evidence.
The reason we take the more conservative position is that compression of the vena cava does not follow a predictable timeline. It varies between women. Waiting until the anatomy suggests it should be a problem means some women will already be experiencing symptoms before they have been told to take any precautions. We would rather give you that information earlier, so you are not caught off guard.
This is not about being overcautious. It is about taking the woman’s experience seriously, not just the anatomy.
What to do instead
There is a wide range of effective exercise you can do during pregnancy without lying flat. Side-lying, seated, standing and inclined positions all allow you to continue training safely and comfortably. A wedge can also make a significant difference, allowing you to remain in a supported inclined position during exercises that would otherwise require lying flat. If you are in a class or working with a trainer and supine exercises are included, ask for a modification. A good instructor will have one ready.
If you do find yourself on your back and feel dizzy, lightheaded or nauseous, roll to your left side straight away. Do not try to sit up quickly.
References
Avery, N.D., Wolfe, L.A., Amara, C.E., Davies, G.A.L. & McGrath, M.J. (1999). Effects of human pregnancy on cardiac autonomic function above and below the ventilatory threshold. Journal of Applied Physiology, 87(4), 1176-1183.
Dalhaug, E.M., Sanda, B., Bø, K., Brown, W. & Haakstad, L.A.H. (2025). Pushing limits: the acute effects of a heavy-load resistance protocol and supine exercise on fetal well-being. BMJ Open Sport & Exercise Medicine, 11, e002639.
Jeffreys, R.M., Stepanchak, W., Lopez, B., Hardis, J. & Clapp, J.F. (2006). Uterine blood flow during supine rest and exercise after 28 weeks of gestation. BJOG: An International Journal of Obstetrics & Gynaecology, 113(11), 1239-1247.
Page, S.M. & Rollins, M.D. (2019). Physiology and Pharmacology of Obstetric Anesthesia. In Pharmacology and Physiology for Anesthesia (2nd ed.). Elsevier.
WHEN Exercise and Pregnancy Guidelines, Version 2 (April 2026). Women’s Health Education Network. www.when.org.au
Developed by the WHEN Clinical Team
Clinically reviewed by the WHEN Clinical Governance Committee
Last reviewed: 02/07/2026
