5.1: Fetal Development of the Heart

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Learning Objectives

By the end of this section, learners will be able to

Differentiate between the embryonic period and the fetal period
Describe the fetal circulatory system and explain the role of the shunts
Trace the development of a fetus from the end of the embryonic period to birth

A developing human is called a fetus from the ninth week of gestation until birth. This 30-week period of development is marked by continued cell growth and differentiation, which fully develop the structures and functions of the immature organ systems formed during the embryonic period. The completion of fetal development results in a newborn who, although still immature in many ways, is capable of survival outside the uterus.¹

5.1.1 The Fetal Circulatory System

When the heart first forms in the embryo, it exists as two parallel tubes derived from mesoderm and lined with endothelium, which then fuse together. As the embryo develops into a fetus, the tube-shaped heart folds and further differentiates into the four chambers present in a mature heart. Unlike a mature cardiovascular system, however, the fetal cardiovascular system also includes circulatory shortcuts, or shunts. A shunt is an anatomical (or sometimes surgical) diversion that allows blood flow to bypass immature organs such as the lungs and liver until childbirth.²

The placenta provides the fetus with necessary oxygen and nutrients via the umbilical vein. (Remember that veins carry blood toward the heart. In this case, the blood flowing to the fetal heart is oxygenated because it comes from the placenta. The respiratory system is immature and cannot yet oxygenate blood on its own.) From the umbilical vein, the oxygenated blood flows toward the inferior vena cava, all but bypassing the immature liver, via the ductus venosus shunt (Figure 5.1.1). The liver receives just a trickle of blood, which is all that it needs in its immature, semifunctional state. Blood flows from the inferior vena cava to the right atrium, mixing with fetal venous blood along the way.³

Although the fetal liver is semifunctional, the fetal lungs are nonfunctional. The fetal circulation, therefore, bypasses the lungs by shifting some of the blood through the foramen ovale, a shunt that directly connects the right and left atria and avoids the pulmonary trunk altogether. Most of the rest of the blood is pumped to the right ventricle and, from there, into the pulmonary trunk, which splits into pulmonary arteries. However, a shunt within the pulmonary artery, the ductus arteriosus, diverts a portion of this blood into the aorta. This ensures that only a small volume of oxygenated blood passes through the immature pulmonary circuit, which has only minor metabolic requirements. Blood vessels of uninflated lungs have high resistance to flow, a condition that encourages blood to flow to the aorta, which presents much lower resistance. The oxygenated blood moves through the foramen ovale into the left atrium, where it mixes with the now deoxygenated blood returning from the pulmonary circuit. This blood then moves into the left ventricle, where it is pumped into the aorta. Some of this blood moves through the coronary arteries into the myocardium, and some moves through the carotid arteries to the brain.⁴

The descending aorta carries partially oxygenated and partially deoxygenated blood into the lower regions of the body. It eventually passes into the umbilical arteries through branches of the internal iliac arteries. The deoxygenated blood collects waste as it circulates through the fetal body and returns to the umbilical cord. Thus, the two umbilical arteries carry blood low in oxygen and high in carbon dioxide and fetal wastes. This blood is filtered through the placenta, where wastes diffuse into the maternal circulation. Oxygen and nutrients from the pregnant person diffuse into the placenta and from there into the fetal blood, and the process repeats.

$Fetal Circulatory system: on the left an umbilical cord connects the blood exchange pathways in the placenta to a baby. Within the child the heart and liver are highlighted and a zoomed in illustration is on the right for both organs. Heart: 1) Oxygenated blood from placenta enters right atrium via inferior vena cava. 2) The foramen ovale allows oxygenated blood in the right atrium to reach the left atrium. 3) The ductus arteriosus connects the aorta with the pulmonary artery, further shunting blood away from the lungs and into the aorta. 4) Mixed blood travels to the head and body, and back, to the placenta via the aorta. Liver: 1) Blood arrives via umbilical vein. 2) The ductus venosus shunts oxygenated blood from the placenta away from the semifunctional liver and toward the heart.$

Figure 5.1.1 Fetal Circulatory System - The fetal circulatory system includes three shunts to divert blood from undeveloped and partially functioning organs, as well as blood supply to and from the placenta.
Image Source: Betts, J. Gordon, Kelly A. Young, James A. Wise, Eddie Johnson, Brandon Poe, Dean H. Kruse, Oksana Korol, Jody E. Johnson, Mark Womble, Peter DeSaix. "Fetal Development", Chapter 28.3 in Anatomy and Physiology. (2013). Chapter 28.3, "Fetal Development". Open Stax, 2013 This work is distributed under a CC BY 4.0 license. Available from Open Stax

5.1.2 Other Fetal Organs

During weeks 9–12 of fetal development, the brain continues to expand, the body elongates, and ossification continues. Fetal movements are frequent during this period but are jerky and not well-controlled. The bone marrow begins to take over the process of erythrocyte production—a task that the liver performed during the embryonic period. The liver now secretes bile. The fetus circulates amniotic fluid by swallowing it and producing urine. The eyes are well-developed by this stage, but the eyelids are fused shut. The fingers and toes begin to develop nails. By the end of week 12, the fetus measures approximately 9 cm (3.5 in) from crown to rump.⁵

Weeks 13–16 are marked by sensory organ development. The eyes move closer together; blinking motions begin, although the eyes remain sealed shut. The lips exhibit sucking motions. The ears move upward and lie flatter against the head. The scalp begins to grow hair. The excretory system is also developing: the kidneys are well-formed, and meconium, or fetal feces, begins to accumulate in the intestines. Meconium consists of ingested amniotic fluid, cellular debris, mucus, and bile.⁶

During approximately weeks 16–20, as the fetus grows and limb movements become more powerful, the pregnant person may begin to feel quickening, or fetal movements. However, space restrictions limit these movements and typically force the growing fetus into the “fetal position,” with the arms crossed and the legs bent at the knees. Sebaceous glands coat the skin with a waxy, protective substance called vernix caseosa that protects and moisturizes the skin and may provide lubrication during childbirth. A silky hair called lanugo also covers the skin during weeks 17–20, but it is shed as the fetus continues to grow. Extremely premature infants sometimes exhibit residual lanugo.⁷

Developmental weeks 21–30 are characterized by rapid weight gain, which is important for maintaining a stable body temperature after birth. The bone marrow completely takes over erythrocyte synthesis, and the axons of the spinal cord begin to be myelinated, or coated in the electrically insulating glial cell sheaths that are necessary for efficient nervous system functioning. (The process of myelination is not completed until adolescence.) During this period, the fetus grows eyelashes. The eyelids are no longer fused and can be opened and closed. The lungs begin producing surfactant, a substance that reduces surface tension in the lungs and assists in proper lung expansion after birth. Inadequate surfactant production in premature newborns may result in respiratory distress syndrome, and as a result, the newborn may require surfactant replacement therapy, supplemental oxygen, or maintenance in a continuous positive airway pressure (CPAP) chamber during their first days or weeks of life. In male fetuses, the testes descend into the scrotum near the end of this period. The fetus at 30 weeks measures 28 cm (11 in) from crown to rump and exhibits the approximate body proportions of a full-term newborn, but still is much leaner.⁸

The fetus continues to lay down subcutaneous fat from week 31 until birth. The added fat fills out the hypodermis, and the skin transitions from red and wrinkled to soft and pink. Lanugo is shed, and the nails grow to the tips of the fingers and toes. Immediately before birth, the average crown-to-rump length is 35.5–40.5 cm (14–16 in), and the fetus weighs approximately 2.5–4 kg (5.5–8.8 lbs). Once born, the newborn is no longer confined to the fetal position, so subsequent measurements are made from head to toe instead of from crown to rump. At birth, the average length is approximately 51 cm (20 in).⁹

For more information

A summary of the stages of pregnancy, as experienced by the pregnant person, and an overview of the stages of development of the fetus can be found online from the Office on Women's Health (OASH.gov).

Exercise $\PageIndex{1}$

At what point in fetal development can a regular heartbeat be detected?

Answer: 8 weeks

Glossary

Bile: A fluid that is made and released by the liver and stored in the gallbladder. Bile helps with digestion. It breaks down fats into fatty acids, which can be taken into the body by the digestive tract.
Carotid arteries: Two principal arteries on both sides of the neck that supply blood to the head and neck; each divides into two branches, the internal carotid artery and the external carotid artery.
Ductus artiosus: A shunt in the fetal pulmonary trunk that diverts oxygenated blood back to the aorta.
Ductus venosus: A shunt that allows oxygenated blood in the umbilical vein to bypass the liver and is essential for normal fetal circulation.
Endothelium: A layer of epithelium that lines the heart, blood vessels, lymph vessels, and the serous cavities of the body.
Erythrocyte: Red blood cell. Mature erythrocytes are non-nucleated, biconcave disks containing HEMOGLOBIN whose function is to transport OXYGEN.
Foramen: An opening, hole, or passage, especially in a bone.
Gestation: The process or period of developing inside the womb between conception and birth.
Iliac arteris: Either of two large arteries originating from the abdominal aorta; they supply blood to the pelvis, abdominal wall, and legs.
Lanugo: Soft, fine hair covering a fetus while inside the uterus.
Meconium: Fetal wastes consisting of ingested amniotic fluid, cellular debris, mucus, and bile.
Mesoderm: The middle germ layer of an embryo, derived from three paired mesenchymal aggregates along the neural tube.
Placenta: A highly vascularized mammalian fetal–maternal organ and major site of transport of oxygen, nutrients, and fetal waste products. It includes a fetal portion (CHORIONIC VILLI) derived from TROPHOBLASTS and a maternal portion (DECIDUA) derived from the uterine ENDOMETRIUM. The placenta produces an array of steroid, protein and peptide hormones (PLACENTAL HORMONES)
Pulmonary trunk: large arterial vessel that carries blood ejected from the right ventricle; divides into the left and right pulmonary arteries
Quickening: fetal movements that are strong enough to be felt by the mother
Shunt: A circulatory shortcut that diverts the flow of blood from one region to another.
Vena cava: The inferior (venous trunk which receives blood from the lower extremities and from the pelvic and abdominal organs) and superior (venous trunk which returns blood from the head, neck, upper extremities and chest) venae cavae.
Vernix caseosa: Waxy, cheese-like substance that protects the delicate fetal skin until birth.

Footnotes

Betts, J. Gordon, Kelly A. Young, James A. Wise, Eddie Johnson, Brandon Poe, Dean H. Kruse, Oksana Korol, Jody E. Johnson, Mark Womble, Peter DeSaix. "Fetal Development", Chapter 28.3 in Anatomy and Physiology. (2013). Chapter 28.3, "Fetal Development". Open Stax, 2013 This work is distributed under a CC BY 4.0 license. Available from Open Stax
Betts et al, 2013, 28.3
Betts et al, 2013, 28.3
Betts et al, 2013, 28.3
Betts et al, 2013, 28.3
Betts et al, 2013, 28.3
Betts et al, 2013, 28.3
Betts et al, 2013, 28.3
Betts et al, 2013, 28.3

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