A baby breathes almost entirely with the diaphragm, the dome-shaped muscle under the lungs, which is why a newborn’s belly rises and falls more than its chest. The diaphragm is doing a second, quieter job too: keeping a little air in the lungs between breaths so they don’t partly collapse. But the muscle a newborn depends on is not yet built for endurance, so the babies who most need it, those born early, are the ones whose diaphragm tires soonest.
Why Babies Breathe With Their Belly
In newborns the ribs run more horizontally than in adults, so the muscles between them can’t raise the ribs effectively.1PubMed Central. Differential Growth and Development of the Upper and Lower Human Thorax Breathing in early life is essentially diaphragm breathing.2PubMed Central. Differential Growth and Development of the Upper and Lower Human Thorax The diaphragm dome also sits higher in a baby’s chest than in older children.3PubMed Central. Changes in rib cage geometry during childhood Most of these shape changes happen in the first 2 years, and as the ribs angle downward, more efficient chest breathing becomes possible.4PubMed Central. Changes in rib cage geometry during childhood5PubMed Central. Differential Growth and Development of the Upper and Lower Human Thorax
An adult can lift the ribs with the muscles between them, a bit like lifting a bucket handle. A baby’s flat ribs have little room to swing upward, so there is little for those muscles to do. Add a soft chest wall, and the chest barely lifts with each breath. The belly is the better place to watch.
When the diaphragm tightens, it flattens and moves down, drawing air into the lungs and pushing the belly out. When it relaxes, it rises again and air flows out.
Newborns breathe faster than adults, often 30 to 60 breaths a minute at rest. Short pauses and an irregular rhythm during sleep are common in healthy newborns. A gently rising and falling belly is a reassuring sign in a calm, comfortable baby. Because the belly needs room to move, pressing or binding it tightly with swaddling or clothing is best avoided.
When a Baby’s Chest Pulls In
As the diaphragm contracts strongly to open the lungs, a newborn’s flexible chest wall can be drawn inward.6PubMed Central. Role of the Chest Wall in Newborn Respiratory Function at Birth The lower rib edges aren’t stiff enough to resist the diaphragm’s pull.7PubMed Central. Role of the Chest Wall in Newborn Respiratory Function at Birth During big breaths, areas higher on the chest can be pulled in too.8PubMed Central. Role of the Chest Wall in Newborn Respiratory Function at Birth This inward movement is fairly common in premature babies, whose chests are especially soft and whose lungs are immature.9PubMed Central. Role of the Chest Wall in Newborn Respiratory Function at Birth It tends to be easier to see during active, dreaming sleep, when the muscles between the ribs relax and the diaphragm can distort the rib cage more.10PubMed. Respiratory load compensation in infants
Visible sinking of the skin between or below the ribs, or at the base of the neck, is a sign that the baby is pulling hard to get air in. A little soft sinking at the lower ribs during dreaming sleep, in a baby who is calm, pink and feeding well, is less concerning, though it is still worth mentioning at the next checkup.
Pulling in that is deep, or that comes with fast breathing, grunting at the end of each breath, or nostrils flaring wide, suggests the baby is working hard and needs to be seen by a doctor promptly. Bluish lips or face, or long pauses in breathing, are emergencies, so call emergency services rather than waiting to see whether things settle.
To check, undress the baby’s chest in good light, since clothing and blankets hide the movement. Look at the skin just below the ribs and above the collarbone, because sinking shows up there first. If you are unsure, a short phone video of your baby’s bare chest and belly while breathing lets the doctor see what you saw, since the pattern often disappears by the time you reach the office.
Holding Air in the Lungs Between Breaths
The air left in the lungs after a normal breath out acts as a reserve that keeps tiny air sacs open. In newborns, the steady background tension of the diaphragm and rib muscles is a major factor in how much air stays in.11PubMed. Importance of inspiratory muscle tone in maintenance of FRC in the newborn When that tension drops, the chest and belly shrink and less air remains.12PubMed. Importance of inspiratory muscle tone in maintenance of FRC in the newborn Even very premature babies can produce strong diaphragm activity right after birth to build up this reserve.13PubMed. Rapid respiratory transition at birth as evaluated by electrical activity of the diaphragm in very preterm infants supported by nasal CPAP Stronger steady diaphragm activity at birth goes with better holding of air as the lungs first fill.14PubMed Central. The Effect of Initial Oxygen Exposure on Diaphragm Activity in Preterm Infants at Birth
Before birth the lungs are filled with fluid, and after birth that fluid is cleared and replaced with air, a huge change for a newborn. The first breaths are the hardest because the tiny air sacs are stuck together and must be popped open, which takes a strong pull. Air left behind after each breath out keeps the sacs open, so the next breath needs much less effort.
A baby whose diaphragm tires can no longer hold that steady tension. The air reserve shrinks, every breath becomes harder, and the baby tires even more. This is one reason support like CPAP (continuous positive airway pressure) is used. It gives a gentle, constant push of air through small prongs at the nose or a mask, which does the job of holding the lungs open. With the lungs held open, the baby’s own diaphragm can spend its effort on moving air, and CPAP lets the baby keep breathing without a breathing tube.
Why a Baby’s Diaphragm Tires Easily
Slow, fatigue-resistant fibers make up only about 1 in 10 of a premature baby’s diaphragm fibers.15PubMed. Developmental pattern of muscle fiber types in human ventilatory muscles Full-term newborns have about 1 in 4, and children over 2 have just over half.16PubMed. Developmental pattern of muscle fiber types in human ventilatory muscles The newborn diaphragm is also thinner and burns oxygen for energy less efficiently, leaving little reserve.17PubMed Central. Respiratory muscle function in the newborn: a narrative review
Full-term babies’ diaphragms can push harder than those of premature babies.18PubMed Central. Diaphragmatic muscle function in term and preterm infants How many weeks a baby was born at predicts diaphragm strength better than sex or time on breathing support.19PubMed Central. Diaphragmatic muscle function in term and preterm infants Immaturity can weaken breathing enough that some preemies need a ventilator.20PubMed. Developmental regulation of molecular signalling in fetal and neonatal diaphragm protein metabolism And when a struggling baby’s diaphragm starts to fatigue, carbon dioxide can build up and the baby can get worse.21PubMed. The consequences of diaphragmatic muscle fatigue in the newborn infant
In the NICU, a tiring baby often breathes faster and more shallowly, moving less air with each breath even though the effort looks greater. Pauses in breathing and dips in oxygen readings may set off monitor alarms more often than before, and nurses watch for that change in pattern.
Staff often increase breathing support when they see early signs, rather than waiting for the baby to tire completely, because a worn-out diaphragm takes longer to recover. When support is lowered too fast, the baby may not manage on their own, which is a common reason a step down has to be reversed. It does not mean the baby is getting worse overall.
The weakness does fade. The share of endurance fibers rises over the first months of life, so the diaphragm becomes more resistant to fatigue as the baby grows. The fiber mix stops changing after about 8 months, so most of that improvement happens early. Preemies gain strength with growth in the weeks and months after birth, not only once they go home, so a baby who struggles now often gets steadily better.
Problems That Can Affect the Diaphragm
In very premature babies on a ventilator, the diaphragm can get measurably thinner within 24 hours.22PubMed. Ventilator-induced diaphragmatic dysfunction in extremely preterm infants: a pilot ultrasound study In one group, about half of ventilated preemies developed diaphragm thinning, usually by the second day.23PubMed. Diaphragm atrophy during invasive mechanical ventilation is related to extubation failure in preterm infants: An ultrasound study That thinning made it more likely the baby would struggle when taken off the ventilator.24PubMed. Diaphragm atrophy during invasive mechanical ventilation is related to extubation failure in preterm infants: An ultrasound study
The phrenic nerve controls the diaphragm, and a difficult delivery can injure it. The injury usually affects one side, most often the right, and the baby struggles to breathe as the healthy side overworks.25PubMed Central. Phrenic nerve palsy: A rare cause of respiratory distress in newborn If both sides are affected, the baby turns blue and needs a ventilator.26PubMed Central. Phrenic nerve palsy: A rare cause of respiratory distress in newborn Only a minority recover on their own, sometimes within about nine days.27PubMed. Neonatal phrenic nerve injury due to traumatic delivery Others have surgery to tighten the floppy side after a median of about 19 days, with good results in 86%.28PubMed. Neonatal phrenic nerve injury due to traumatic delivery
Nerve injury to the diaphragm can occur alongside weakness in one arm after a difficult delivery, since the nerves involved sit close together in the neck. Feeding poorly or growing slowly can also occur. If breathing trouble follows a shoulder or arm injury at birth, ask the team whether the diaphragm has been checked. These signs can have other causes, so they are a reason to ask, not a diagnosis.
About 1 in 2,500 babies is born with a hole in the diaphragm that lets belly organs move into the chest, a condition called congenital diaphragmatic hernia. The lungs grow too small and their blood vessels are narrowed, causing breathing trouble.29PubMed. Fetal lung and diaphragm development in congenital diaphragmatic hernia Diaphragm strength in these children tends to be low in the first year and in the normal range by about age 5.30PubMed. Diaphragmatic function in infants and children with congenital diaphragmatic hernia: a cross-sectional study
A bedside ultrasound lets doctors watch the diaphragm move and thicken with each breath, which shows whether it is working and whether one side lags. The scan is painless and uses no radiation, so it can be repeated as often as needed. Normal values for newborns now exist, so a measurement can be compared with what is expected for a baby that age. Repeated scans in a baby on a ventilator can show whether the muscle is thinning over time and help the team decide how much support to give.
How Position and Feeding Change the Work
Recovering premature babies used less diaphragm effort lying on their tummy than on their back.31PubMed. Effect of prone and supine positioning on the diaphragmatic work of breathing in convalescent preterm infants Lying on the back with the head tilted up eased effort about as much.32PubMed. Effect of prone and supine positioning on the diaphragmatic work of breathing in convalescent preterm infants In sick babies in intensive care, lying on the tummy kept the diaphragm from overworking.33PubMed. Comparison of diaphragmatic thickness fraction between prone and supine positioning in preterm and sick term neonates: a prospective study
In the hospital, tummy lying is done with constant monitoring of breathing and heart rate, so staff can react right away if something changes. At home, babies should always be put to sleep on their back, because tummy sleeping raises the risk of sudden infant death. Supervised tummy time while the baby is awake is still recommended, as it helps build strength, and it should end if the baby falls asleep. Before discharge, ask the NICU team how to position your baby at home, since the hospital position may be different from what is safe in a crib.
Feeding changes the workload too. In premature babies, tube feeds made the diaphragm work harder.34PubMed. The effect of gavage feeding on the mechanics of the lung, chest wall, and diaphragm of preterm infants A full stomach can reduce the air left in the lungs.35PubMed. The effect of gavage feeding on the mechanics of the lung, chest wall, and diaphragm of preterm infants And each swallow closes the airway for about 1 second.36PubMed. Coordination of breathing and swallowing in human infants
A baby who breathes hard may manage smaller, more frequent feeds better than large ones. Short pauses to breathe during feeds are normal, because the baby needs to catch up after each swallow. Letting the baby rest between bursts of sucking, and stopping if the baby seems worn out, keeps feeding from taking over the baby’s energy. Coughing, a color change or exhaustion during feeds should be raised with the doctor, as it may mean the baby cannot yet coordinate sucking, swallowing and breathing.