Researchers who studied 4D scans of 15 healthy fetuses also said they think yawning is a developmental process which could potentially give doctors a new way to check on a baby's health.

While some scientists have previously suggested that fetuses yawn, others disagree and say it is nothing more than a developing baby opening and stretching its mouth.

But writing in the journal PLOS ONE on Wednesday, British researchers said their study was able to clearly distinguish yawning from "non-yawn mouth opening" based on how long the mouth was open.

The researchers did this by using 4D video footage to examine all the times when fetuses opened their mouths.

Nadja Reissland of Durham University's department of Psychology, who led the study, said the function and importance of yawning in fetuses is still unknown, but the findings suggest it may be linked to fetal development and could provide a further indication of the health of the unborn baby.

"Unlike us, fetuses do not yawn contagiously, nor do they yawn because they are sleepy," she said. "Instead, the frequency of yawning in the womb may be linked to the maturing of the brain early in gestation."

The study was carried out on eight female and seven male fetuses from 24 to 36 weeks gestation. The researchers found that yawning declined from 28 weeks and that there was no significant difference in how often boys and girls yawned.

(Reporting by Kate Kelland, editing by Paul Casciato)

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