Babies yawn in the womb, says new research, but this doesn't mean that they are sleepy.

Researchers from Durham and Lancaster Universities have now found that fetal yawning can be an indication that the fetus is healthy.

4D scans of babies studied by the researchers reveal that yawing is a normal developmental process that can show how well the fetus is growing. Fetal yawn was earlier thought to be normal mouth opening. But, the present study shows that babies in the womb indeed yawn.

"Unlike us, fetuses do not yawn contagiously, nor do they yawn because they are sleepy. Instead, the frequency of yawning in the womb may be linked to the maturing of the brain early in gestation," said Dr Nadja Reissland, of Durham University's Department of Psychology, lead researcher of the study.

For the study, fifteen healthy fetuses; eight female and seven male, were scanned four times at 24, 28, 32 and 36 weeks gestation. Researchers found that yawning declined from 28 weeks and that the frequency of yawing was same for boys and girls.

"The results of this study demonstrate that yawning can be observed in healthy fetuses and extends previous work on fetal yawning. Our longitudinal study shows that yawning declines with increasing fetal age," said Reissland.

Why fetuses yawn is still unclear. Researchers say that yawning can be a sign that the fetus is developing normally.

"Given that the frequency of yawning in our sample of healthy fetuses declined from 28 weeks to 36 weeks gestation, it seems to suggest that yawning and simple mouth opening have this maturational function early in gestation," Reissland said.

The study is published in the journal PLOS ONE.