Cardiovascular Development At a Glance
| Weeks | Days | Somites | Length in mm | Cardiac Events |
| 1-2 | 0-20 | 1 | 1.5 | No heart or great vessels |
| 3 | 20 | 2 | 1.5 | Cardiogenic plate |
| 3 | 21 | 5 | 1.5 | Endocardial tubes |
| 4 | 22 | 10 | 2 | Fusion of endocardial tubes |
| 4 | 23 | 12 | 2 | Single median cardiac tube,first contraction ( ineffective) |
| 4 | 25 | 17 | 2.5 | Cardiogenic loop |
| 4 | 26 | 20 | 3 | Single atrium |
| 5 | 29 | 25 | 4 | Bi-lobed atrium |
| 5 | 30 | 26 | 4 | Beginning of circulation |
| 5 | 31 | 28 | 4.8 | Septum primum |
| 5 | 35 | 7.5 | A-V orifice, 3 chamber heart | |
| 6 | 36 | 8.5 | Septum secundum | |
| 6 | 39 | 10 | Complete inferior septum | |
| 6 | 40 | 10.5 | Septation of bulbus and ventricle | |
| 6 | 42 | 13 | Divided truncus arteriosus | |
| 7 | 49 | 20 | 4-chambered heart, Absorption of pulmonary veins |