Foramen Ovale
There is a foramen ovale (oval hole) between the right and left atrium in the fetus. After birth, this should close. If it does not, this is an atrial septal defect (hole in the heart). In the fetus, the right atrium pumps blood into the left atrium, bypassing the pulmonary circulation (which is useless in a fetus). In an adult, a septal defect would result in flow in the reverse direction - from the left atrium to the right - which will reduce cardiac output, potentially cause cardiac failure and in severe or untreated cases, death.
Read more about this topic: Left Atrium