From the Research
Approximately 18-25% of blood from the right atrium flows through the foramen ovale in a fetus, as estimated from the distribution of human fetal combined cardiac output during the second half of pregnancy 1. This represents a significant portion of the fetal circulation that bypasses the lungs, which are not yet functioning for gas exchange. The foramen ovale is a critical shunt in fetal circulation that allows oxygenated blood from the placenta to move directly from the right atrium to the left atrium, bypassing the pulmonary circulation. This adaptation is necessary because the fetus receives oxygenated blood from the placenta rather than through its own lungs. The remaining percentage of blood from the right atrium flows into the right ventricle and then mostly through the ductus arteriosus into the aorta, with only a small amount going to the non-functioning lungs. After birth, the foramen ovale typically closes as pulmonary circulation is established and pressure in the left atrium exceeds that in the right atrium, forming the fossa ovalis in the interatrial septum.
Some key points to consider:
- The foramen ovale has a limited capacity to increase its volume blood flow at near-term gestation 2.
- The proportion of blood flow through the foramen ovale decreases from 34% to 18% from 20 to 30 weeks of gestation 1.
- The human fetal pulmonary circulation has an important role in the distribution of cardiac output 1.
- The foramen ovale is an essential part of fetal circulation, allowing oxygenated blood from the umbilical cord to bypass the lungs 3.
It's worth noting that while the exact percentage of blood flow through the foramen ovale may vary, the most recent and highest quality study available 1 provides the best estimate of this value.