Before a baby is born, when it is still a fetus safely in the womb of its mother, it receives oxygen-rich blood from the mother’s placenta via the umbilical cord. During birth, the baby transitions from the support of the placenta to autonomous breathing and circulation, a magical process involving major circulatory and hemodynamic changes.
Newborn and adult circulation
In babies and adults, all the blood flows from the right side of the heart through the lungs, to be oxygenated, back to the left side of the heart. The left side then pumps the oxygenated blood into the body, to provide oxygen and nutrients to our cells. De-oxygenated blood then returns back into the right side of the heart.
Fetal circulation is different to the circulation of a newborn baby and an adult. In the fetus, not much blood passes through the lungs. The fetus is not breathing air and the lungs are filled with fluids. Rather, it receives oxygen-rich blood from the mother’s placenta via the umbilical cord. In fetal circulation, the placenta mainly supplies blood to the left atrium, through an anatomical opening called the foramen ovale.
Aside for the patent foramen ovale, there is another important connection, the patent ductus arteriosus, the connection between the pulmonary artery and the aorta, allowing blood to travel through the pulmonary artery to bypass the lungs and go straight into the aorta. Blood can bypass the lungs because of high vascular resistance in the pulmonary arteries, allowing the blood to more easily flow into the aorta.
In the fetus, oxygenated blood flows through the umbilical vein to the liver and passes through the ductus venosus before joining with the inferior vena cava. The blood will enter the right atrium and then either go into the right ventricle or it will go through the patent foramen ovale into the left atrium, before being pumped in the left ventricle and into the (ascending) aorta. The blood that is pumped from the right ventricle can go through the pulmonary artery to enter the lungs, but mainly bypasses the lungs straight into the (descending) aorta via the patent ductus arteriosus. Through the aorta, blood flows into the fetal body and back to the placenta via the umbilical arteries.
Transition during birth3-6
After birth, when the baby takes its first breaths, its lungs fill with air. With these first breaths, the lungs start to expand and the alveoli in the lungs are cleared of fluids. Pulmonary vascular resistance reduces, allowing a fast increase of pulmonary blood-flow. This leads to a steady change of blood-flow: it is no longer the mother’s placenta, but the pulmonary blood flow that provides all the blood to the left atrium, stimulating the foramen ovale to close. The umbilical cord can now be safely clamped, thereby separating the baby from its placenta. The baby’s first breaths are crucial to redirect blood-flow to start passing through its lungs, triggering a steady transition to autonomous circulation.
Umbilical cord clamping3-6
Around 1 in 10 babies do not breathe at birth. Around the world, this affects over 35,000 newborns every day. Todays medical convention still prescribes to cut the umbilical cord soon after birth, in order to provide urgent care. The placental circulation is cut off before the baby’s lungs are filled with air.
The first problem of clamping the umbilical cord so soon, is that blood, that was returning from the baby back into the placenta, is now all redirected through the baby’s body. Systemic vascular resistance increases and as a result, there is a very rapid increase in arterial pressure. This increases by at least 30% in only 4 heartbeats.
Secondly, the left side of the heart is deprived of venous return or preload. The blood-flow from the placenta into the heart is cut off while at the same time the pulmonary blood-flow remains low. The baby’s heart has less blood to pump, its heart rate will drop and cardiac output greatly decreases by up to 50% in only 60 seconds, thereby reducing the supply of vital oxygen to the baby’s organs, especially the brains.
The rapid increase in blood-pressure and the reduced oxygen supply to the organs, can cause injury, especially to the baby’s brains and intestines.
Only after the baby’s lungs fill with air, redirecting the blood-flow through the lungs, the left atrium will be supplied with sufficient blood, to restore cardiac output
Placental transfusion and circulation15
By keeping the umbilical cord intact, placental circulation and blood-flow from and to the baby is maintained for some minutes after birth. This blood is re-distributed to the baby’s organs, gradually transfusing blood volume from the placenta to the baby, delivering oxygen and important nutrients like iron.
In term neonates, the blood-flow from the placenta contributes to an additional 80-100ml circulating blood volume. This adds around 30% circulating blood-volume, including around 20-30mg/kg iron.
Research shows (ref) that gravity has little to no effect on placental transfusion and circulation. The main driver for placental circulation is the baby’s breathing, causing a pressure difference that “pulls” blood into its own circulation.
Time Based Cord Clamping7-11
A lot of clinical research has been done in recent years to research the benefits of delaying cord clamping. Most studies compare immediate cord clamping with the benefits of Time Based Cord Clamping, clamping the cord after a fixed time of 60-120 sec. Time Based Cord Clamping is also referred to as Delayed Cord Clamping. Meta-analyses shows a number of benefits of Time Based Cord Clamping:
There is no evidence of any adverse impact or harm for the baby or the mother.
Physiological-based cord clamping means clamping the umbilical cord at a time when the baby is fully stable and breathing on its own. The timing of cord clamping is determined by the baby’s own condition instead of a fixed time.
Allowing the baby to aerate its lungs before clamping the cord, leads to a gentle switch from the oxygen-rich blood-flow from the placenta to autonomous breathing and blood flow. This prevents a rapid increase in blood-pressure and sustains heart rate and cardiac output and thus the supply of vital oxygen to the baby’s organs.
A study in preterm lambs comparing cord clamping before and after the onset of ventilation, shows the rise in pulmonary blood-flow and the stable arterial pressure and heart rate in the ventilated group compared to the unventilated group.3
Avoiding bradycardia and hypoxemia in the first minutes after birth, is associated with a reduced risk of brain hemorrhage (IVH) or death.13
The positive effects of physiological-based cord clamping for preterm babies has for the first time been shown in a recently published feasibility study. In total 33 babies born <35 weeks’ gestational age received physiological-based cord clamping with the Concord Birth Trolley at Leiden University Medical Center.
In this study, heart rate remained stable around the moment of cord clamping and no bradycardia was observed after clamping the cord. Oxygen levels in the blood (SpO2) increased at a faster pace when comparing with preterm infants receiving immediate cord clamping and the duration of hypoxemia was shorter in the first 10 minutes after birth.
Recently, a large multicenter randomized trial started, to investigate the effects of physiological-based cord clamping: the ABC3 trial, led by Prof. Arjan te Pas, from Leiden University Medical Center.
ABC is the abbreviation for “Aeration, Breathing then Clamping”, referring to a “new” ABC protocol for neonatal resuscitation, based on physiological-based cord clamping.
The objective of the trial is to investigate the effect of physiological-based cord clamping compared to time based cord clamping (of 60 sec) without stabilization. Around 700 preterm born infants <30 weeks gestational age will be recruited in this study, that is performed by a consortium of Dutch and international hospitals.
The primary outcome of ABC3 is an absolute increase in intact survival at hospital discharge, defined as survival without cerebral injury (IVH >grade 1 and/or PVL >grade 1 and/or cerebral infarction) and/or necrotizing enterocolitis (NEC >grade 1).