Introduction: Knowledge of morphological features of the fetal heart is necessary to achieve an accurate diagnosis of possible malformations as well as other conditions that may cause physiological alterations in the fetal heart progressing to cardiac failure and fetal death, if not opportunely corrected. Clinical anatomy of the true interatrial septum is treacherous, difficult and its unfamiliarity can cause many serious complications. Materials and Methods: 40 formalin preserved fetal hearts at gestational age from 14 to 40 weeks were dissected through midline thoracotomy and cutting open the pericardial sac. Foramen ovale diameter and septum primum excursion in the left atrium were measured in millimeter. Result: In gestational age group of 14­20 weeks (14 fetuses), foramen ovale diameter (FOD) was 2.3 mm in male and 2.2 mm in female while septum primum excursion (SPE) was 2 mm in both. In 21­30 weeks (16 fetuses), FOD was 2.5 mm in both while SPE was 2.2 in male and 2.1 in female. In 31­40 weeks (10 fetuses), FOD was 32 mm in male and 3.8 mm in female while SPE was 4 mm in male and 3.6 mm in female. Conclusion: A sound knowledge about the normal anatomy and development of the fetal heart has become absolutely necessary to know the fetal echocardiography and new surgical techniques for correction of prenatally diagnosed cardiac malformations.