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A new baby’s heart begins to develop as early as week five of pregnancy (3 weeks after conception) when the tube of muscle forms that will someday be the baby’s heart. This first primitive structure doesn’t have any of the separate chambers or valves that will appear in later weeks and months.

By weeks six and seven, the fetus has the beginnings of a heartbeat, but the heart itself is still lacking its highly developed structures. The development, however, is rapid. By week 22, a stethoscope can pick up the sound of the fetal heartbeat.

A normal heart has two sides, with two chambers on each side and a thick muscular wall called the septum dividing these two sides. The heart is also divided horizontally, between the upper chambers (atria) and lower pumping chambers (ventricles). Finally, a number of large veins and arteries are connected to the heart, including:

  • Vena cava veins—These bring unoxygenated blood back to the heart from the body.
  • Pulmonary artery—This large artery carries blood from the right side of the heart to the lungs where the blood will receive oxygen.
  • Pulmonary veins—These veins carry freshly oxygenated blood back to the heart from the lungs.
  • Aorta—This main artery is connected to the left ventricle; its job is to carry oxygenated blood from the heart out to the body.
  • Coronary arteries—These tiny arteries feed the heart muscle itself.

The heart’s development inside the womb is an amazing process. Because the fetus develops inside an amniotic sac, surrounded by fluid, the baby doesn’t need to breathe air so the lungs aren’t used. As a result, there’s no reason for the heart to pump blood through the lungs to receive oxygen. (Instead, fetal oxygen comes through the placenta.)

Before birth, a special blood vessel called the ductus arteriosus connects the aorta to the pulmonary artery. This extra vessel allows blood to flow straight from the pulmonary artery to the aorta, bypassing the lungs. The ductus arteriosus closes after birth. If it doesn’t, it’s known as a patent ductus arteriosus and will require medical intervention.

Another difference in the fetal circulatory system is the fetus has a hole in the muscular septum between the heart’s upper chambers. This hole is called the foramen ovale. This, too, serves to bypass the lungs by allowing blood to flow between the right and left atria. Like the ductus arteriosus, the foramen ovale closes after the baby is born. If it doesn’t, it is called a patent foramen ovale and will require evaluation.

When a baby is born and the first breaths are taken, a remarkable process begins that shifts the heart from its fetal structure to a normal adult structure. The ductus arteriosus and foramen ovale both close over the first few days of life, effectively separating the right side of the heart and the left side of the heart.

Reviewed by Dr. Sara Connolly, August 2019

Takeaways

  • The fetal circulatory system works differently than a newborn’s because a fetus doesn’t breathe through its lungs while in the womb.
  • The developing fetal heart bypasses the lungs with a ductus arteriosus, which is the blood vessel connecting the aorta and pulmonary artery, and foramen ovale, a hole between the heart’s upper chambers.
  • The ductus arteriosus and foramen ovale normally close after birth so that the lungs can take over.

References

  1. National Institutes of Health, Medline Plus. Fetal Development.
  2. Kids Health. If Your Child has a Heart Defect.
  3. National Heart Lung and Blood Institute. What is Patent Ductus Arteriosus?
  4. Texas Heart Institute. The Fetal Heart.
  5. American Heart Association. About Congenital Heart Defects.

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