Scans show frequency of action, thought to be linked to growth of nervous system, drops as birth date nears if all is well

Ultrasound scans that catch unborn babies yawning in the uterus may help doctors monitor the normal development of children in the womb.

The idea comes from researchers who used footage of foetuses at different gestational stages to show that healthy babies yawned less as they approached their birth dates. Scientists at Durham and Lancaster Universities said that while it remained unclear why unborn babies yawned, the movement may be related to the maturation of their central nervous systems.

"It may be that, in order to get part of the brain to mature in the correct way, you need a certain stimulus, and yawning might be that stimulus," said Nadja Reissland, a psychologist who specialises in foetal development at Durham University.

The prospect of monitoring foetal yawns as an indicator of a baby's health is supported by other studies in the field. Previous work has found evidence to suggest that some medical conditions might affect the yawning patterns of growing foetuses. In 2002 researchers at New York Downtown hospital noticed that unborn babies with anaemia displayed "unusual bursts" of yawning.

Reissland and others used 4D ultrasound scans to capture a rapid sequence of images for eight female and seven male foetuses at 24, 28, 32 and 36 weeks of gestation. All the unborn babies were healthy and were scanned for 20 minutes. The researchers went through the images frame by frame and counted the yawns and other movements made by the babies. Over 58 scans, the team recorded 56 yawns and 27 other mouth movements. Yawns can be distinguished from other movements because a yawning mouth opens more slowly than it closes.

When the scientists analysed the images they found similar yawning rates in boys and girls. But, more surprisingly, yawning decreased steadily from nearly twice every 10 minutes in foetuses at 24 weeks gestation, to none at 36 weeks. The research appears in the journal Plos One.

"Unlike us, foetuses 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," Reissland said.

"Given that the frequency of yawning in our sample of healthy foetuses 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."

Last year the same group of scientists, working with James Cook University Hospital in Middlesbrough, published work that showed two unborn baby girls developed complex facial expressions while in the womb.

According to ultrasound scans of the foetuses, expressions that looked like crying and laughing developed between 24 and 35 weeks.

The scientists now hope to scan more unborn babies to confirm the downward trend in yawning with gestational age, and to move on to studies of other foetuses, such as those exposed to drugs, or diagnosed with specific medical conditions. The team has ethical approval to scan unborn babies in mothers who smoke, but no funding to pursue the work.