The heart tube elongates on the right side, looping and becoming the first visual sign of left-right asymmetry of the body.

[3] The heart derives from embryonic mesodermal germ layer cells that differentiate after gastrulation into mesothelium, endothelium, and myocardium.

Heart induction occurs in the anterior mesoderm during gastrulation through interactions with adjacent endoderm (both extra-embryonic and definitive) mediated primarily by endogenous inhibitors of WNT signaling such as DKK1.

[6][2] In the splanchnopleuric mesenchyme on either side of the neural plate, a horseshoe-shaped area develops as the cardiogenic region.

As embryonic folding starts, the two endocardial tubes are pushed into the thoracic cavity, where they begin to fuse together, and this is completed at about 22 days.

The growth of the brain and the cephalic folds push the oropharyngeal membrane forward, while the heart and the pericardial cavity move first to the cervical region and then into the chest.

The tube starts receiving venous drainage in its caudal pole and will pump blood out of the first aortic arch and into the dorsal aorta through its polar head.

[7] The myocardium thickens and secretes a thick layer of rich extracellular matrix containing hyaluronic acid which separates the endothelium.

[7] The heart tube continues stretching and by day 23, in a process called morphogenesis, cardiac looping begins.

Since there is no evolutionary pressure on the heart and inner organs for bilateral symmetry, these body parts are excluded from the twisting and remain asymmetric.

This movement is caused mainly by the left to right shunt of blood, which occurs in the venous system during the fourth and fifth week of development.

The growth consists of two tissue masses actively growing that approach one another until they merge and split light into two separate conduits.

In these places, the cushions will help in the formation of auricular septum, ventricular conduits, atrio-ventricular valves and aortic and pulmonary channels.

Coalescence of these perforations will form the ostium secundum (second opening), which allows blood to flow freely from the right atrium to the left.

When the right of the atrium expands due to the incorporation of the pole of the sinus, a new fold appears, called the septum secundum.

The passage between the two atrial chambers consists of a long oblique slit through which blood flows from the right atrium to the left.

[13] Initially, a single pulmonary vein develops in the form of a bulge in the back wall of the left atrium.

The division begins in the common ventricle where a furrow in the outer surface of the heart will appear the interventricular foramen eventually disappears.

This closure is achieved by further growth of the muscular interventricular septum, a contribution of trunk crest-conal tissue and a membranous component.

Myocytes that were obtained in the primitive heart tube, start beating as they connect together by their walls in a syncytium.

The heartbeat begins in the region of the pacemaker which has a spontaneous depolarization time faster than the rest of myocardium.

In the first trimester, cardiac activity can be visualized and the fetal heart motion quantified by obstetric ultrasonography.

A study of 32 normal pregnancies showed that fetal heart motion was visible at a mean human chorionic gonadotropin (hCG) level of 10,000 UI/L (range 8650–12,200).

[19] Obstetric ultrasonography can also use Doppler technique on key vessels such as the umbilical artery to detect abnormal flow.