While the fetus is still in the womb, the lungs are full of amniotic fluid and are not used for breathing. Waste removal, as well as oxygen exchange, occurs through the placenta. At birth, the baby's body must suddenly adjust to new processes for sustaining life in its new environment.

FETAL STRUCTURES:
Blood is pumped through the umbilical cord and placenta for waste excretion and oxygen exchange, bypassing the lungs in a fetus. There are some structures in the fetal vascular system that make this type of blood movement possible:

• Ductus arteriosus -- This is a link between the blood vessels that normally supply blood to the lungs for gaseous exchange, and the aorta, which is the main vessel supplying oxygenated blood to the body.
• Foramen ovale -- An opening between the atria (top chambers) of the heart. It facilitates movement of oxygenated blood through the fetus' body.
• Ductus venosus -- A blood vessel leading from the liver to a larger vessel called the inferior vena cava.
• Umbilical vein -- The vessel leading from the umbilical cord to the liver. This vessel delivers oxygenated blood to the fetus' body.
• Umbilical arteries -- The vessels leading from the fetal arterial system to the umbilical cord. Deoxygenated blood is carried through these vessels.

While the fetus is in the womb, blood flows through the placenta. There, waste products are removed and oxygen is picked up through an exchange process with the mother's blood vessels. This is done through membranes, and normally the fetal blood does not come in contact with maternal blood.

The cleansed and oxygenated blood flows through the large umbilical vein through the liver and the ductus venosus to the inferior vena cava and finally into the heart. A small amount is pumped to the lungs, but a majority flows through the foramen ovale and the ductus arteriosus into the aorta where the blood is then distributed throughout the body.

The deoxygenated blood is then returned through the umbilical arteries to the placenta, where waste products are removed and the blood is reoxygenated.

CHANGES AT BIRTH:

During a vaginal birth the lungs are compressed, removing much of the fluid that formerly filled the lungs. Once the head emerges from the vaginal canal, the healthcare provider removes fluid from the nasal and oral cavities with a suction bulb, to help clear the airway if needed.

As soon as the baby emerges, the healthcare provider clamps and cuts the cord and takes stimulatory measures to help the baby take his first breath (this is why the baby is sometimes "spanked" to encourage crying and therefore breathing).

There are many motivators for the baby to take the first breath:

• temperature change
• light stimulation
• physical stimulation
• lack of oxygen supply (since the cord is cut)
• negative pressure in the chest cavity resulting from the recoil of the chest after exiting the vaginal canal

Once the cord is cut and the baby takes his first breath, a number of changes occur in the newborn's vascular system. The increased oxygen in the lungs causes a decrease in blood flow resistance to the lungs. There is also an increase in the blood flow resistance of the body's vessels.

The lack of umbilical blood flow, decreased pulmonary vascular resistance, and increased systemic vascular resistance cause a closure of the foramen ovale. All the other vascular (blood supply) peculiarities of the fetus begin to constrict and eventually turn into supporting ligaments with no vascular responsibilities.

Now the newborn's blood flow is internalized, gas exchange occurs in the lungs, waste products are removed through urination and defecation, and nutrition is obtained through the digestive system. The newborn undergoes dramatic changes in the processes that sustain life, and these new methods take some time to fully adapt and become permanent.