For women in the third trimester of their pregnancy, lying down on their back may place stress on the fetus, which could increase the risk of stillbirth in certain cases, a small new study from New Zealand suggests.

However, experts say that it's too early to make recommendations based on the findings, and that pregnant women do not need to change the way they lie down as a result of the study.

"It is not possible to draw a firm link between maternal position and stillbirth risk from this study and further robust research is needed," Hannah Knight, a spokesperson for the Royal College of Obstetricians and Gynaecologists in the United Kingdom, said in a statement. "Women should sleep in a position that is comfortable for them," Knight said.

In the study, the researchers at the University of Auckland monitored 29 healthy, pregnant women and their fetuses while the women lay down in different positions β€” on their right or left side, and on their back β€” for 30 minutes at a time. The women were 35 to 38 weeks pregnant. 

The researchers found that, 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.

In addition, when the women lay on their back, the fetuses were more likely to switch from being in a more active state to the sleep-like state, compared to when the women lay on their left side.

The researchers also looked at the fetuses' heart rate variability, which is a measure of the variations in the interval between heartbeats. They 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, compared to those women in the study who lay down on their left side. Lower heart rate variability is known to precede fetal distress, the researchers said.

Overall, the researchers said that their findings suggest that, for the women studied, lying on the back is mildly stressful for the baby, and the fetus adapts to this by switching to a state where less oxygen is consumed.

They speculated that in certain 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's imposed by the mother lying on her back. However, none of the women in the study had a stillbirth.

"The supine position may be disadvantageous for fetal wellbeing and in compromised pregnancies, may be a sufficient stressor to contribute to fetal demise," the researchers wrote in the Nov. 22 issue of the The Journal of Physiology.

All of the women gave birth at full term, and neither the newborns nor their mothers experienced any complications.

β€œIt is important that women are not unnecessarily alarmed by the results of this small study of 29 women, none of whom had a stillbirth," Knight said. She also noted that the study measured the occurrence of a sleep-like state in the fetuses, "which has little evidence of any association with hypoxia, brain damage or stillbirth."

This study is not the first to suggest that there's a link between the mother's position while she is lying down and a risk of stillbirth. In a 2011 studyof about 500 women, the same group of researchers found that women who slept on their back had an increased risk of stillbirth compared to those who slept on their left side. In that study, the researchers calculated that the risk of stillbirth for women who slept on their back or their right side was about 4 out of 1,000, compared to about 2 out of 1,000 in women who slept on their left side.

The researchers noted that in their new study, they looked at the effect of maternal positions only while the women were awake, and only for a very short period of time, said study researcher Dr. Peter Stone, a professor of maternal fetal medicine at the University of Auckland. More research is needed to examine the physiological effect of staying in certain sleeping positions overnight, Stone said.

Original article on Live Science.