Pathological details are lacking of various techniques used for interatrial defect (IAD) creation in lesions requiring mixing or with left atrial hypertension. Therefore, the morphology of the IAD created by different interventional techniques is described. The atrial septa of ten euthanized piglets (seven at <3 days and three at 2 weeks of age) were surgically exposed. In pigs of both ages, a needle-created communication was dilated with angioplasty, cutting, and cryoplasty balloons. A stent was also implanted in a newborn pig. By way of a patent foramen ovale (PFO) in newborns, angioplasty and septostomy balloons and a stent created IADs. The morphology of the IAD was directly imaged and the size measured. Newborn piglets had noticeably thinner atrial septa, and a PFO was still present. Static balloon dilation created a circular IAD equivalent to the balloon diameter (3 and 6 mm) in both the created defects of pigs of both ages and the neonatal PFO. A 3-mm cutting balloon (CB) produced a 2-mm triangular IAD (corresponding to atherotomes) in pigs of both ages. Premounted stents, by way of a created defect or PFO, led to circular IADs equivalent to balloon diameter. The 3-mm cryoplasty balloon created a large 5 × 4-mm IAD in the newborn pig; however, the IAD measured only 2 × 1-mm at 2 weeks. By way of a neonatal PFO, a septostomy balloon (inflated to 2 cc) created a 3-mm circular IAD. In neonatal piglet hearts, static balloon angioplasty, CBs, and stents created a predictable IAD. Cryoplasty balloons created highly variable defects.