During embryogenesis at about two weeks, a primitive atrium begins to be formed. It begins as one chamber, which over the following two weeks becomes divided by the septum primum into the left atrium and the right atrium. The interatrial septum has an opening in the right atrium, the foramen ovale, which provides access to the left atrium; this connects the two chambers, which is essential for fetal blood circulation. At birth, when the first breath is taken fetal blood flow is reversed to travel through the lungs. The foramen ovale is no longer needed and it closes to leave a depression (the fossa ovalis) in the atrial wall.