The Newborn Transition: From Placenta to Breathing

Birth requires the baby to change from placental oxygen supply to breathing air. The lungs expand, blood flow changes and temporary fetal circulation pathways begin to close. Most babies complete this transition without major intervention. Some need immediate support.

Before birth, the placenta supplies oxygen The fetal lungs are filled with fluid and are not used for air breathing. Oxygen reaches the fetus through the placenta and umbilical vein. Blood vessels in the fetal lungs remain relatively constricted. This creates high resistance to pulmonary blood flow. The foramen ovale and ductus arteriosus allow much of the circulation to bypass the lungs. The ductus venosus directs part of the umbilical venous blood towards the heart. These pathways are normal adaptations to fetal life. The first breaths After birth, sensory, chemical and temperature changes stimulate breathing. The baby's first effective breaths expand the lungs. Air enters the small air sacs called alveoli. Fluid begins moving out of the air spaces into surrounding tissues, blood vessels and lymphatic channels. The process continues after the first breath. Some remaining fluid can contribute to temporary rapid breathing. Surfactant keeps the lungs open The inside surface of each alveolus is moist. Without protection, surface tension would make the air sacs difficult to keep open. Surfactant reduces surface tension and supports stable expansion. It is produced by specialised cells within the developing lungs. Premature babies may have insufficient surfactant. This can cause neonatal respiratory distress syndrome and may require breathing support and replacement surfactant. Pulmonary resistance falls As the lungs expand and oxygen levels rise, pulmonary blood vessels relax. Resistance to blood flow through the lungs falls sharply. More blood leaves the right ventricle and travels through the pulmonary arteries. Blood then returns from the lungs through the pulmonary veins to the left atrium. The lungs now take over gas exchange from the placenta. This change transforms the circulation from the fetal pattern towards the newborn pattern. Cord clamping changes pressure Before the cord is clamped, the placenta remains part of the circulation. Clamping removes the low resistance placental circuit. Systemic vascular resistance rises. At the same time, increased pulmonary blood flow raises pressure within the left atrium. Pressure within the right atrium falls as umbilical venous return ends. These pressure changes help close the fetal shunts. The foramen ovale begins closing Before birth, pressure in the right atrium directs blood through the foramen ovale. After breathing begins, increased pulmonary blood returns to the left atrium. Left atrial pressure becomes higher than right atrial pressure. A flap over the foramen ovale is pushed closed. This is initially a functional closure. The tissues may fuse later. A small opening can remain in some healthy adults without causing symptoms. The ductus arteriosus constricts The ductus arteriosus connects the pulmonary artery to the aorta before birth. After birth, increased oxygen and reduced circulating placental prostaglandins encourage it to constrict. Functional closure usually begins during the early newborn period. An open ductus can remain necessary in some congenital heart conditions. Doctors may deliberately keep it open with prostaglandin medicine until specialist treatment occurs. A patent ductus arteriosus can also persist unintentionally, particularly in premature babies. The ductus venosus closes The ductus venosus carried umbilical venous blood towards the heart. Cord clamping stops umbilical venous flow. The ductus venosus then constricts and closes. More blood from the digestive organs subsequently travels through the liver in the usual postnatal pattern. Transition is a process Circulatory transition does not occur as one instantaneous switch. Breathing, pressure changes and vessel closure occur over different timescales. A baby can appear well while anatomical closure remains incomplete. Some transitional openings normally persist for hours or days before closing functionally. Persistent fetal circulation becomes a problem only when it disrupts oxygen delivery or cardiovascular function. Delayed cord clamping When immediate resuscitation is not required, delaying cord clamping allows continued placental transfusion. Current newborn resuscitation guidance emphasises delayed cord clamping, including for many preterm babies. When urgent intervention cannot be performed safely with the cord intact, earlier clamping may be necessary. The priority is effective transition and timely care rather than one rule regardless of circumstances. Keeping warm A newborn loses heat rapidly through evaporation, contact with cooler surfaces, air movement and nearby cold objects. The baby has a large surface area compared with body mass. Newborns cannot shiver effectively like adults. They produce heat partly through brown fat, a specialised heat generating tissue. Drying, warm towels, skin to skin contact and an appropriate room temperature help prevent hypothermia. Preterm or unwell babies need additional thermal support. Low temperature can worsen breathing and glucose control. Skin to skin contact Skin to skin contact can support temperature, cardiorespiratory stability, feeding and parent infant interaction. It should occur when both the parent and baby are clinically stable. The baby's face and airway must remain visible and unobstructed. A drowsy or sedated parent requires supervision. Skin to skin contact should never replace assessment of an unwell baby. Glucose after birth Before birth, glucose arrives continuously through the placenta. Cord clamping stops this supply. The newborn must use stored glycogen, produce glucose and begin feeding. Healthy term babies usually adapt without routine glucose testing. Babies who are premature, small, large, unwell or born after maternal diabetes may have greater risk of neonatal hypoglycaemia. Low glucose can cause feeding difficulty, jitteriness, lethargy, abnormal temperature or seizures. These signs are non specific and require clinical assessment. Beginning to feed Newborn feeding depends on alertness, breathing, coordination and anatomical function. Sucking, swallowing and breathing must be coordinated. Colostrum is produced in small volumes suited to the early newborn stomach. Frequent feeding helps stimulate later milk production. Formula feeding also requires responsive pacing, correct preparation and safe storage. A baby too breathless or sleepy to feed needs urgent assessment. Feeding difficulty can be an early sign of infection, respiratory illness, neurological problems or low glucose. Passing urine and meconium Most newborns pass urine and meconium during the first day. Meconium is the dark, sticky first stool. Timing varies, but failure to pass urine or meconium can indicate obstruction, dehydration or another condition. Green vomiting is not normal meconium passage. It can indicate intestinal obstruction and requires emergency assessment. Bilirubin and jaundice Red blood cells are broken down after birth. This produces bilirubin. The newborn liver processes bilirubin less efficiently than an older child's liver. Mild jaundice is common after the first day. Jaundice within the first 24 hours is abnormal and needs urgent investigation. Very high bilirubin can affect the brain. Jaundice may be harder to recognise on darker skin. The whites of the eyes, gums and blanched skin should be examined in good light. Vitamin K Newborns have low vitamin K stores. Vitamin K is required to make several clotting proteins. Without supplementation, a small number of babies develop vitamin K deficiency bleeding. An intramuscular vitamin K injection is routinely offered after birth. Oral regimens may be available but usually require repeated doses. Parents should receive information about the purpose, method and implications of declining. Newborn assessment Immediate assessment considers breathing, heart rate, muscle tone, colour, response to stimulation and temperature. The Apgar score records selected observations after birth. It helps communicate the baby's early condition and response to support. It does not predict intelligence or provide a complete long term prognosis. A low score requires assessment of the cause rather than interpretation in isolation. Newborn resuscitation Most newborns breathe without advanced resuscitation. Some need drying, warmth, positioning or gentle stimulation. When the baby is not breathing effectively, lung inflation and ventilation are the priority. Chest compressions or medicines are required much less often. Teams prepare before high risk births so support can begin without delay. Parents should receive an explanation and debrief when resuscitation occurs. Prematurity A premature baby faces additional transitional challenges. The lungs may contain less surfactant. The skin is thinner, increasing heat and fluid loss. Glucose stores and feeding coordination may be limited. The brain, gut and immune system remain less mature. Special neonatal care supports each function while development continues. Prematurity does not produce one predictable outcome. Gestation, birth condition, illness and response to treatment all matter. Persistent pulmonary hypertension In some babies, pulmonary vascular resistance remains abnormally high after birth. This is called persistent pulmonary hypertension of the newborn. Blood continues bypassing the lungs through fetal pathways. Oxygen levels can remain dangerously low. Possible causes include infection, meconium aspiration, lung disease or abnormal lung development. Treatment may include oxygen, ventilation, medicines and specialist intensive care. Infection can disrupt transition Newborn infection can cause breathing difficulty, temperature instability, feeding problems or reduced responsiveness. Young babies may have serious infection without fever. Risk factors include maternal infection, group B streptococcus and prolonged rupture of membranes. A clinician assesses the combination of risk factors and clinical signs. Antibiotics may be started promptly when infection is sufficiently likely. Not every baby with one risk factor requires the same treatment. Newborn screening and examination A complete newborn physical examination is offered within 72 hours. It includes general assessment and specific checks of the eyes, heart, hips and testes where relevant. Hearing screening is offered during the early newborn period. A blood spot sample is taken from the heel to screen for selected serious conditions. Screening identifies babies who may need further testing. A positive screen is not the same as a confirmed diagnosis. A normal screen does not exclude every possible childhood condition. Parent observations matter Parents often notice changes between professional checks. Important changes include poorer feeding, unusual sleepiness, a weak or continuous cry, altered colour, abnormal breathing, reduced wet nappies, repeated vomiting, and a change in responsiveness. One symptom may be non specific. A newborn's condition can change quickly, so concern should be taken seriously.

The newborn transition depends on effective breathing, falling pulmonary resistance and coordinated closure of fetal circulation pathways.