Piontelli
A. Fetal Yawning in Development of
Normal Fetal Movements. The fisrt 25 weeks of
gestation. Spinger. 2010.
135p.
Introduction
The development of yawning, a movement which
has been reported in humans and many vertebrates
from fetal stages to old age remains poorly
understood [3]. According to Provine
[4] human are unique because in contrast
to other species, for example rats, where males
yawn more frequently than females [5],
in humans both sexes yawn equally often. There
is however, a lack of research on fetal
yawning.
Research suggests that there is a U-shaped
developmental progression to yawning in that
premature infants yawn more frequently than term
babies [6] and primary school children
yawn more frequently than kindergarten children
[7]. Most research on yawning, a
movement defined as mouth opening with the jaw
dropping, relates to the contagious nature of
yawning. Provine [4] suggests that yawns
are so infectious that simply thinking or
reading about yawning results in a yawn in
around 60% of observations [8].
Interestingly, children are immune from the
contagious nature of yawning until around five
years of age [9] hence not only yawning
frequency but also the social context of
yawning, such as contagious yawning, has a
developmental component which is as yet
unexplained. Contagious yawning however cannot
be the reason for fetal yawning. Another theory
which potentially explains fetal yawning
suggests that yawning is related to central
nervous system (CNS) arousal modulation and
hence related to waking motility patterns
[6]. This is supported by evidence that
cortisol levels are increased during stress and
fatigue as well as yawning [10].
However, others disagree arguing that fetuses do
not yawn because they feel sleepy'' [11 p:
36]. Even though the relationship between
the neural network of mouth, tongue and
respiratory movements is not well understood,
some research suggests that the function of
yawning in fetuses might lie in
activity-dependent brain maturation
[12].
In summary, yawning has been reported
from the end of the first trimester,
[1]. Sherer, Smith, & Abramowicz
[13] described yawning in a case study
at 20 weeks gestation. Roodenburg, Wladimiroff,
van Es, & Prechtl [14] observed 9
fetuses in 2D imaging who only occasionally
yawned but they were able to ascertain that jaw
movements increased up to 28 weeks and then
declined. Yigiter, & Kavak, [15], in
a cross sectional study, report no significant
changes in mouthing, yawning and sucking. One
reason for varied findings could be due to a
lack of a precise definition of yawns compared
with mouth opening. McMagnus, Devine, &
Brandsetter [16] go so far as to argue
that definitions of yawning are so varied that
what has been labelled a yawn can be just a
mouth opening or repeated set of mouth openings
rather than yawns. Indeed, in terms of a measure
of fetal neurological development [17],
yawning and mouth opening are not distinguished.
Provine ([4] p532) suggests a general
definition of a yawn consisting of jaws open in
a wide gape, a deep inward breath followed by a
shorter exhalation and a closing of the jaws.
Robust differentiation between wide mouth
opening and a yawn in contrast requires a
dynamic definition of yawning [18].
Hence, it is essential to differentiate between
wide mouth opening with the jaw dropping and
yawning in terms of the timing of these mouth
movements either being labelled a yawn or mouth
stretch. One dynamic definition proposed by
Petrikovsky, et al. [18] is a
''prolonged wide opening of the mouth followed
by a quicker closure of the mouth''. Petrikovsky
et al. [18] used this definition of
yawning in 2-D ultrasound examinations of fetal
mouth movements. Applying this dynamic
definition to their data they found a mean of 5
yawns in 18 of 22 healthy fetuses. In contrast,
the nine anaemic fetuses studied yawned more
frequently with a mean of 12 yawns observed in
the 1 hour period. The present longitudinal
study examined the dynamics of yawning compared
with non-yawn mouth opening in 15 healthy
fetuses, 8 girls and 7 boys, observed four times
over the second and third trimester of
pregnancy. We expected that if yawning is a
developmental process, then the frequency of
yawning might change during gestation. Given
previous research on human yawning we also did
not expect to identify sex differences.
Discussion and Conclusion
There are a number of hypotheses explaining
the ubiquitous behaviour of yawning
[24]. One of them is that yawning is a
response to elevated levels of carbon dioxide or
depressed levels of oxygen in the blood which
has been shown to be a false association
[2]. The results of this study
demonstrate that yawning can be observed in
healthy fetuses and replicates previous studies
with 2-D images. In contrast to previous
research we could also show that although
healthy fetuses vary in the frequency of yawns
observed overall, the repeated measures design
allowing an observation of the same fetuses at
24 to 36 weeks gestation in 4 weekly intervals
showed that in healthy fetuses the frequency of
yawning declines over time. Specifically, in our
longitudinal study following 15 fetuses, we
observed yawns when the fetus was active at 24
weeks but not in the similarly active fetus at
36 weeks (Figure 3). This finding corroborates
work by Giganti et al. [6] who
videotaped 12 healthy infants ranging in age
from 31 to 40 weeks (post gestational age) and
recorded 1.10+20.7 yawns for the infants in a 24
hour period which decreased with age to zero
yawns.
Hence, this research could support the
suggestions that yawning is related to CNS
maturation e.g. [25&endash;26] rather
than arousal modulation. Furthermore, supporting
other results on similar yawning frequency in
human males and females, in our research we did
not find any sex differences in yawning
frequency. In summary, the importance and
function of yawning is still unclear. Some
researchers found an association of yawning with
neurological functioning e.g.
[25&endash;27]; others found a
relationship between yawning and Parkinson's
disease [28]. Yet other research argued
that yawning has a thermo-regulative function
[29]. A further hypothesis states that
blood cortisol levels might be the cause for
yawning [30] or that yawning could be
due to the increase of venous blood to the heart
of the fetus [18]. A developmental
function of yawning could be related to jaw
movements, which are important in the prenatal
and postnatal development of the cartilage in
the temporo-mandibular joint, enabling normal
mouth movements [31]. In contrast to
these medical reasons for yawning others argue
that yawning has a communicative function
[32]. In our sample, we can exclude the
communicative function of yawning. However,
other hypotheses could be supported. Given that
the frequency of yawning in our sample of
healthy fetuses declined from 24 to 36 weeks
gestation, it is possible that yawning and
simple mouth opening have a maturational
function early in gestation. Although, yawning
and simple mouth opening have the same
trajectory shape over age it is notable that the
yawning rate is just over double the nonyawning
rate. In order to exclude the potential function
of cortisol in yawning, in future research it
would be important to measure maternal cortisol
levels at the time of observing fetal yawns.
