Pregnant mums are anxious about a study showing sleeping on your back increases your stillbirth risk.

There are a lot of things to be concerned about when you are pregnant. You worry about what you eat, how you exercise, what medicines you take, what chemicals you are exposed to, but just in time to make us even more stressed experts are now telling us we also need to be aware of how we sleep.

The sleep warning has been prompted by a New Zealand study that found women in the third trimester of their pregnancy who sleep on their back may place stress on their unborn baby which could increase the risk of stillbirth in certain cases.

Researchers from the University of Auckland monitored 29 healthy, pregnant women between 35 to 38 weeks and their fetuses while the women lay down in different positions, on their right or left side, and on their back for 30 minutes at a time.

The Journal of Physiology reports when the women lay on their backs or their right side, the fetuses were more likely to be in a sleep-like state that's associated with using less oxygen, compared to when the women lay on their left side. The fetus adapted to the mildly stressful situation by consuming less oxygen.

The researchers also studied the fetuses' heart rate variability, and found a link between those pregnant women in the study who lay on their back and a reduction in heart rate variability in the fetus. Researchers said that lower heart rate variability is known to precede fetal distress.

Researchers speculated in some cases, such as when the fetus is already deprived of adequate oxygen due to other factors, the fetus might not be able to adapt to the extra stress that is imposed by the mother lying on her back.

The researchers wrote in The Journal of Physiology "The supine position may be disadvantageous for fetal wellbeing and in compromised pregnancies, may be a sufficient stressor to contribute to fetal demise.”

"Our controlled study found that lying on your back can add extra stress to the baby, contributing to the risk of stillbirth," Peter Stone, Professor of Maternal Fetal medicine from the University of Auckland and lead investigator of the study said. "The risk is likely to be increased further in women with underlying conditions."

"We have only looked at the effect of maternal positions for a short period of time while the mother is awake.”

He then stressed that “further research is needed to see the effect of staying in certain maternal sleeping positions overnight."

Is it the first time we’ve been told this?

No. A 2011 British Medical Journal Article studied 500 women and found that women who slept on their back had an increased risk of stillbirth compared to those who slept on their left side.

They found the risk went from two out of 1,000 in women who slept on their left side to four out of 1,000.

Should women who sleep on their backs be losing sleep over this study?

No. Hannah Knight, a spokesperson for the Royal College of Obstetricians and Gynaecologists in the United Kingdom, said in a statement, "It is not possible to draw a firm link between maternal position and stillbirth risk from this study and further robust research is needed."

"Women should sleep in a position that is comfortable for them."