Women who go to sleep on their back in late pregnancy are more likely to have stillbirths. Now, research from a University of Auckland-U.K. collaboration helps to explain why, showing a decreased supply of oxygen to the fetus.

“Sleep on side when baby’s inside,” is New Zealand’s official advice to women in late pregnancy, based largely on University of Auckland research that helped to spread that message around the world over the past decade. 
In the latest study, women with healthy pregnancies underwent MRI (magnetic resonance imaging)  scans at Auckland City Hospital lying on their backs and their sides. The scans showed the reduction in blood flowing to the uterus when mum lies on her back. On average, oxygen delivery to the fetus fell 6.2%, the paper just published in the Journal of Physiology showed.

This study pushes ahead research in the area by quantifying the physical effects that seem to be linked to the heightened risk of stillbirth. 

``This helps us to answer questions about the rationale for the ‘sleep on your side’ advice,” said Sophie Couper, a University of Auckland medical school student who played a key role in the research. ``There’s still more to discover, but now we understand more about why sleeping on your back can be bad.”

Sophie Couper, a University of Auckland medical school student who played a key role in the research

The study relied on collaboration with researchers at King’s College London and University College London (UCL) who developed an innovative placental MRI algorithm.

Healthy fetuses may be fine when their mothers sleep on their backs. But, for babies who are vulnerable, a mother’s sleeping position could be the extra factor that contributes to a stillbirth. 
Each year, in New Zealand, about 160 babies are stillborn in the last three weeks of pregnancy, according to the “Sleep on Side” website. A 10% decrease in late stillbirth might be achieved if all women followed the sleeping advice, according to the website.
Scientists know that unborn babies are less active, a warning sign for potential ill health and risk of still birth, when a mother in late pregnancy is lying on her back. The lack of activity may be a sign of a lack of oxygen, and that a baby is changing its behavior to use as little energy as possible.  

The MRI scans showed reductions in blood and placental oxygen concentration when the women lay on their backs. On average, blood flow to the uterus decreased by 23.7%.
The study relied on a new placental MRI technique, Diffusion-Relaxation Combined Imaging of the Placenta (DECIDE), developed by Professor Anna David, Dr Rosalind Aughwane (UCL) and Dr Andrew Melbourne (King’s College London). The new technique can differentiate between maternal and fetal contributions to blood flow  as well as oxygen delivery across the human placenta.

“It seems that sleeping in the wrong position is an additional stressor that may be too much for a fetus that is already vulnerable, resulting in an increased risk of a still birth,” said Professor Peter Stone, of the School of Medicine in the Faculty of Medical and Health Sciences at the University of Auckland.  “Vulnerable fetuses include those which are smaller than usual and which may already be getting limited oxygen.” 
In Auckland, the study brought together expertise from the university’s departments of obstetrics and medical imaging, and its Auckland Bioengineering Institute.

Link to study: https://physoc.onlinelibrary.wiley.com/doi/10.1113/JP280569