Figure 1. Click on the image to view ultrasound footage which shows a fetus breathing and hiccupping
...that a fetus practices breathing for months before birth?
Fetal breathing movements are episodic and irregular, interspersed with periods of apnea , and in humans they become detectable by ultrasound at 10-11 weeks gestation [2, 3]. Fetal breathing movements become more regular and uniform as gestational age increases . Breathing movement frequency increases until the 10 weeks before birth, when periods of apnea increase [5, 6].
General fetal movement has been used for centuries to monitor fetal well being. Hippocrates believed that movement began 70-90 days after conception, and that males moved more vigorously than females . Paré, a 16th century French surgeon and obstetrician, recognized that absence of fetal movement could be a sign of intrauterine death . The first recorded observations of intrauterine breathing movements in a human fetus were made in 1888 by the German doctor von Ahlfeld. He and his student, Weber, used a kymograph to record 'periodic, rhythmic intrauterine fetal movements' that they attributed to breathing motions . Many of their contemporaries dismissed the theory that these were fetal breathing movements because no one had ever visually observed such movements in a fetus. In 1911 another German scientist, Reifferschied, published simultaneous kymograph recordings of fetal breathing movements, maternal respiration, and maternal pulse . He noted that these actions all occurred at different rates. Although the scientific community remained skeptical, his measurements would later prove to be remarkably accurate.
It was not until the 1970s that two landmark studies answered the question of whether fetal breathing movements normally occurred in vivo. The work was performed by two groups - Merlet and her colleagues from France, and British scientists led by Dawes. In 1970 and 1972, these groups published papers with data obtained from fetal lambs in utero. The authors showed that breathing movements normally occur during gestation and do so at irregular intervals [1, 9]. Later work with animals proved that eliminating fetal breathing movements results in immature, underdeveloped lungs [10, 11].
More recent in vitro studies have investigated the signaling pathways between fetal breathing movements and lung development. Fetal breathing movements stretch the lungs and move fluid in and out of the lungs. Mechanical stretch upregulates the release of serotonin via mechano-sensitive channels , which promotes differentiation of epithelial cells. Stretch also increases epithelial cell proliferation [13, 14] and stimulates secretion of lung surfactant lipids from type II epithelial cells . Parathyroid hormone-related protein (PTHrP) is the product of a stretch sensitive gene expressed by the lung which is essential for normal lung development . In the absence of PTHrP, lipofibroblasts spontaneously transdifferentiate into myofibroblasts . Lipofibroblasts are the source of the lipids necessary for surfactant synthesis. Over-distension of alveolar epithelial cells caused loss of PTHrP mRNA , resulting in a less mature lung phenotype and mirroring the effects of ventilation in premature infants.
While the importance of fetal breathing movements is now widely accepted, the underlying mechanisms of lung development dependent on these movements are still being revealed with both in vivo and in vitro studies. These scientific results will hopefully lead to improvements in patient care, as they show that fetal breathing movements in utero are vital for postnatal lung function.
Author: Rebekah Morrow
Chief editor: Donna Cioffi, Ph.D., November, 2012
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