System Development in the Fetus
From its beginning as a tiny embryo to its birth, the human fetus undergoes faster growth and more incredible transformations than it will experience in a whole lifetime outside the womb. Every small milestone in its development is a huge leap forward, and each of its complex, life-sustaining systems depends on proper formation of vital components at critical stages.
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Early Stages
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The first two stages of pregnancy include the fertilization of the egg and the implantation of the embryo. About three weeks after conception, the embryo forms a disc with three layers: an outer layer called the the ectoderm, a central layer called the mesoderm and an inner layer called the endoderm. Each of these layers contains the building blocks for separate systems in the human body.
Circulatory System
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The heart and circulatory system of a fetus begin forming in the mesoderm layer. The mesoderm combines with specialized tissue to make the umbilical cord, which sets the stage for blood, oxygen and nutrient flow from the placenta to the fetus. Six weeks after conception, blood vessels are formed, and the fetus's heart begins pumping blood. Eight weeks after conception, the circulatory system is solidly in place and almost completely functional. At around 12 weeks, the liver begins to produce red blood cells. At around 23 weeks, the fetus's bone marrow begins to produce white blood cells.
Nervous System
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Development of a rudimentary neural tube begins in the first month of pregnancy, in the ectoderm layer of the embryo. By the eighth week after conception, the neural tube has closed and the brain, spinal cord and peripheral nerves have developed. The fetus develops muscle sense, shows coordinated movement and is sensitive to light and touch.
At about 3 1/2 months gestation, nervous reflexes like sucking, swallowing and breathing begin to develop. At 16 weeks, nerve connections from the ears to the brain are formed, and the fetus can hear. In the seventh month, the fetus develops reflex responses such as pupil dilation and the startle reflex.
Digestive System
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The digestive system begins as a basic tube lined with mucous membranes. The tube is formed in the endoderm at about the third week after conception. By the eighth week after conception, the the fetus's bladder, intestines, stomach, liver and pancreas are formed.
The production of urine in the bladder and secretions in the liver and pancreas begin in the 13th week. At around the 16th week, bile begins to collect in the intestines, and the first meconium is produced.
The Respiratory System
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The respiratory system also develops from the endosperm layer. The same membrane-lined tube that forms the intestines forms the lungs and trachea, beginning at around six weeks after conception.
The fetus receives all of its oxygen from the umbilical cord, so there is no blood flow to the lungs. Blood bypasses the lungs by way of a hole that's supported by a shunt. At birth, the hole closes and the lungs become fully functional. The nervous system does prepare the baby for the act of breathing by prompting rhythmic reflex breathing movements.
From 21 to 23 weeks, the lower areas of the lungs form, and at 24 weeks air sacs develop. By 28 weeks, the lungs are ready for gas exchange, although they lack maturity.
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