The foramen ovale (hole in the atrial septum which allows for R -> L shunting in utero) remains open until around 3 months of age. Even then, however, it may not ever close and I've read in many texts that about 30% of adults have a PFO. Usually clinically insignificant unless somebody develops a venous thromboembolism which could slip through and cause an arterial stroke.
-Sarah Reece-Stremtan (pediatric anesthesiology fellow in Washington DC)
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