Marvel at the development of the fetus inside the mother's womb.

In 3 weeks of pregnancy, the fetal heart is beating although it's just a tube that must fuse together to transform into four-chambered heart. A fetus undergoes stages of development until its organs become fully developed. Adult and fetal cardiovascular systems are different in the sense that the fetus depends on the mother for its oxygen, received through the placenta. At the moment of birth, as the newborn takes its first breath, is the only time the cardiovascular system is put to the test.