The heart starts as a simple tube that begins to beat around the fourth week of pregnancy, then folds and divides into four chambers by about week eight. Small errors in this process cause congenital heart defects, the most common type of birth defect.
Fetal circulation
Before birth the placenta supplies oxygen, so most blood bypasses the lungs. The foramen ovale lets blood cross from the right to the left atrium, and the ductus arteriosus carries blood from the pulmonary artery to the aorta.
Changes at birth
With the first breaths, pressure in the lungs falls and left atrial pressure rises, which pushes the foramen ovale closed. The ductus arteriosus usually closes within the first days of life.
Holes and open channels
Ventricular septal defect (VSD) is the most common defect. VSD, atrial septal defect and patent ductus arteriosus let blood shunt from the left side to the right, which can overload the lungs and cause a murmur.
Cyanotic defects
Some defects send blood low in oxygen to the body, such as tetralogy of Fallot and transposition of the great arteries. These babies can look blue (cyanotic) and often need surgery early in life.
At the bedside
Newborn pulse oximetry screening compares the right hand with a foot to catch critical defects. A reading under 95% or a difference of more than 3% between the two sites calls for further evaluation.
Related guides
Written by Making Synapses. Topic reference: OpenStax, Anatomy and Physiology 2e (no text adapted).