The first breath: starting breathing at birth and birth asphyxia
- Mechanism
- Exam priority: medium
Before birth the lungs are full of fluid and the fetus only practises breathing. At birth, cold, touch and mild asphyxia trigger the first breath.
Also called: Neonatal transition, Perinatal asphyxia
What it is
The fetal lung is a fluid-filled organ that the fetus exercises with intermittent breathing movements. At birth it must clear that fluid, open the alveoli against surface tension and start continuous breathing within about a minute. Sensory stimuli, the mild hypoxia and hypercapnia of labour and cord clamping drive the first breaths. A newborn tolerates lack of oxygen longer than an adult, but prolonged asphyxia injures the brain.
Key facts
- Lung fluid production late in gestation is several mL/kg/h; the fluid keeps the lungs at a volume that drives their growth.
- Causes of hypoxia during delivery: cord compression, placental abruption, excessive uterine contractions and maternal depression by drugs or anaesthesia.
- Newborn hypoxia tolerance comes from cardiac glycogen and anaerobic glycolysis, lower brain metabolism and left-shifted HbF.
- Asphyxial brain injury often affects the deep grey matter: thalamus, basal ganglia and brainstem.
- Lung compliance approaches the normal newborn level within about the first hour.
How it works, step by step
- Labour squeezes the fetus and the umbilical circulation, causing mild hypoxia, hypercapnia and acidosis that stimulate chemoreceptors.
- At delivery, cold air on wet skin, touch, light and sound give a burst of sensory input to the respiratory centre.
- Cord clamping removes placental factors (such as prostaglandin E₂ and adenosine) that inhibit fetal breathing.
- The first deep inspiration opens fluid-filled airways; fluid is pushed into the interstitium and cleared by lymph and capillaries over hours.
- Surfactant lowers surface tension and some air stays in the lungs at end-expiration, creating the functional residual capacity.
- Lung expansion and higher alveolar O₂ dilate pulmonary arterioles; pulmonary resistance falls and pulmonary blood flow rises.
- If breathing fails, PaO₂ falls and PaCO₂ rises; this usually triggers breathing within another minute, unless the respiratory centre is depressed.
- Ongoing asphyxia: rapid breathing → primary apnea → gasping → secondary apnea, with falling heart rate and blood pressure.
Newborn transition (term, approximate)
| Parameter | Value | Note |
|---|---|---|
| Time to first breath | seconds | regular rhythm usually within 1 min |
| Respiratory rate | 40–60 /min | |
| Heart rate | 110–160 /min | |
| Preductal SpO₂ at 1 min | ≈ 60–65% | rises over several minutes |
| Preductal SpO₂ at 10 min | ≈ 85–95% | |
| Apgar score timing | 1 and 5 min | repeated at 10 min if < 7 |
Approximate values for a healthy adult at rest; each laboratory sets its own ranges.
Clinical notes
- Apgar score (heart rate, respiratory effort, tone, reflex irritability, colour) records how the transition is going; it does not by itself diagnose asphyxia.
- Respiratory distress syndrome of the preterm: surfactant deficiency means each breath is like the first; alveoli collapse at end-expiration.
- Meconium passed during fetal hypoxia can be aspirated with gasping, blocking airways and inactivating surfactant.
- Hypoxic–ischemic encephalopathy follows severe perinatal asphyxia: acidosis, low Apgar scores, encephalopathy and multi-organ injury.
Test yourself
Think of the answer, then tap the question to see it.
A term baby born by elective caesarean without labour breathes at 80/min with mild retractions, improving over two days. What is the mechanism?
Retained lung fluid: without labour, catecholamines do not switch the lung epithelium to Na⁺ absorption through ENaC, so fluid clears slowly (transient tachypnea of the newborn).
Why is the first breath harder than the second?
The first breath must push fluid out of the airways and open collapsed alveoli against surface tension; afterwards air and surfactant keep a residual volume, so less pressure is needed.
How do fetal and newborn responses to hypoxia differ?
Hypoxia suppresses fetal breathing movements but stimulates breathing after birth, once peripheral chemoreceptors reset and placental inhibitors are gone.
What distinguishes primary from secondary apnea in a newborn?
Primary apnea comes first and responds to stimulation; after a period of gasping, secondary apnea follows and needs assisted ventilation.
Connections
Educational physiology content; not a diagnostic or treatment guide and no substitute for a health professional's advice. Values are approximate references. No doses are given.
Sources: Apgar score: MedlinePlus Medical Encyclopedia (MedlinePlus, US National Library of Medicine, 2026); Transient tachypnea - newborn: MedlinePlus Medical Encyclopedia (MedlinePlus, US National Library of Medicine, 2026); Neonatal respiratory distress syndrome: MedlinePlus Medical Encyclopedia (MedlinePlus, US National Library of Medicine, 2026).
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Representative model, for educational purposes only.