One of the most exciting moments of pregnancy for parents-to-be is the sight or sound of the beating fetal heart. By the time the heart is able to be heard when amplified by fetal Doppler, around nine to ten weeks of pregnancy, (see reference 5) the heart is already fully formed, with four chambers, two atria and two ventricles, just like an adult heart. The heart and circulatory system develop from the middle layer of embryonic cells, the mesoderm.
Primitvie Heart Tube
When the heart first develops, it's nothing more than a tube formed from the fusion of two other tubes. By day 22 or 23 after conception, which is week five of pregnancy, the heart begins to beat and can be seen on vaginal ultrasound as a tiny flicker. The developing heart, nothing more than a tube, consists of three layers and three specific areas. The three areas, the cranial, caudal and bulbis cordis, develop into different parts of the aorta, the largest artery in the body, and the ventricles. The layers, the cardiac jelly and cardiac mantle layers, of which there are two, will differentiate into the myometrium and epicardium of the heart.
Two and Three Chambered Heart
As the heart rapidly expands within the next day, it assumes an S shape as it loops over on itself and bends to the right, known as d-looping, creating a primitive area where the ventricle will grow. In the two chambered stage, the cardiac jelly acts as a valve between the atria and ventricular areas, according to the Virtual Cath Lab. The atria divide first, creating a three chambered heart consisting of two atria, the top chambers, and one ventricle, the lower chamber.
Four Chambered Heart
By the end of week eight of embryonic development, or week ten of pregnancy, the heart has formed, with two atria and two ventricles and two great blood vessels to carry the blood from the heart, the aorta and pulmonary artery, John McNulty, PhD, of the Loyola University Medical Center explains.
Fetal circulation differs from circulation after the baby is born, because a fetus doesn't use lungs to breathe. Most fetal blood doesn't pass through the lungs, but instead is shunted through the foramen ovale, which allows highly oxygenated blood to pass from the right and left ventricle, the University of California at Berkeley Department of Molecular and Cell Biology explains. After birth, the foramen ovale closes and oxygenated blood enters the left atrium from the lungs. Two other shunts, the ductus arteriosus and the ductus venosus, also close after birth.